Batch create, batch delete for symbols

[Lobankovanastia]

I have many markers on the map and it was visible how do they appear one after another, so I implemented batch create and batch remove for symbols. I would be happy to contribute.
See:
single:
batch:

[m0nac0]

@Lobankovanastia Thank you for this PR, this is a very useful feature and has already been requested in the past! (#210)
I just left a suggestion on how we might be able to simplify the Android code a little bit, I basically just pasted your code together to reduce the number of new classes ;)
(I personally feel like it would be nicer to have a slightly more complex MapboxMapController at the benefit of less additional classes, but that's just my two cents, and others might think differently about it.)
Let me know what you think about it.

[Lobankovanastia]

Thank you for feedback @m0nac0, @TimothySealy ! I applied requested changes. I would also prefer to keep only symbol#add with functionality of addAll, symbol#remove with functionality of removeAll on android side and on Dart side keep the current interface(addSymbol, addSymbols) and for adding one symbol transmit array with one element. Would you approve?

[m0nac0]

@Lobankovanastia Great work!
I think your second suggestion is reasonable as long as it doesn't impact performance. I added a hacky "performance test" button to the symbols example page with the following code:

FlatButton(
  child: const Text('performance test'),
  onPressed: () {
    final stopwatch = Stopwatch()..start();
    controller.addSymbol(
      SymbolOptions(
          geometry: LatLng(-33.852, 151.211),
          iconImage: "airport-15"),
    );
    stopwatch.stop();
    print(
        'adding single symbol executed in ${stopwatch.elapsed}');
    final stopwatch2 = Stopwatch()..start();
    controller.addSymbols([
      SymbolOptions(
          geometry: LatLng(-33.852, 151.211),
          iconImage: "airport-15")
    ]);
    stopwatch2.stop();
    print(
        'adding list with one element executed in ${stopwatch2.elapsed}');
  },
),

On Android, this gives me negligible differences of below one millisecond, which is totally ok. I guess it'll be similar for iOS, but I haven't been able to run the performance test on iOS. (If you want to make sure, you could also run the above test on iOS yourself)
So feel free to go ahead with that change as well!

[Lobankovanastia]

@m0nac0, on ios test gives similar results:
flutter: adding single symbol executed in 0:00:00.003253
flutter: adding List with one element executed in 0:00:00.000646
flutter: adding single symbol executed in 0:00:00.000689
flutter: adding list with one element executed in 0:00:00.000575
flutter: adding single symbol executed in 0:00:00.008279
flutter: adding list with one element executed in 0:00:00.001663

I tested the example app, it works as expected on both platforms. Please take a look at the final code.

[m0nac0]

@Lobankovanastia I just left two more small suggestions, everything else is looking good! Also thank you for running the performance test.

[m0nac0]

Suggested change

	case "symbol#add": {
	case "symbols#addAll": {

Could we please keep the "symbol#addAll" or possibly even better: "symbols#addAll"?
This also makes it clear that adding symbols is currently implemented differently than adding circles and lines (which we might want to update at a later time to support batch editing, as well).
Sorry for not thinking about this earlier.

[m0nac0]

Since this isn't really doing anything relevant anymore, couldn't we just replace its usage(s) with new SymbolBuilder()?

[Lobankovanastia]

Sure! Sorry for delay. I have updated PR.

[tobrun]

This is great @Lobankovanastia! would you be able to rebase on top of head of master? With introduction of web support we now have a slightly different codestructure.

[m0nac0]

Great work!
Side note: on Android I'm now sometimes seeing newly added symbols appear, disappear again for a very short time and then appear permanently, so they're blinking once just after they're added. This is very subtle though and not a blocker. Just wanted to mention it in case you happened to now the reason.

[Lobankovanastia]

@m0nac0, thank you for approval! Regarding the issue, I have also seen that, but I checked the master branch - same there

[m0nac0]

@Lobankovanastia Good to know it's not related to this PR then, probably just a coincidence that I noticed it today.

[Lobankovanastia]

I created an issue with blinking