Fix rendering of gtk menus

[MattSturgeon]

• Fixed the linux getMenuParent to return the correct parent of the menu
• Fixed AddMenu and AddMenuItem to show the added menu/item

Menus now render natively when Global.js allows, however commands are not yet attached and the API is still not implemented properly. All this includes so far is fixes to make it work at the most basic level.

Note: I'm learning GTK and C especially for this, so I might need a little bit of hand holding with the NativeMenuModel integration. I'll try to look into that soonish, but let me know if anyone else is working on it already :)

[ingorichter]

@MattSturgeon F.Y.I.: I did a lot of work related to the native menues in this branch irichter/native-menues-linux. There is some cleanup required to make this production ready, but I basically integrated the NativeMenuModel already.

[eyelash]

I did some work on native menus as well (including the integration of NativeMenuModel). Is there somewhere a list of who is working on what because I find this duplication of work quite frustrating.

[redmunds]

@eyelash Thanks for helping out with Brackets. Sorry for any duplication of work.

There is no formal list of who's working on what. In the Hot to Hack on Brackets wiki page the Number `1 item is "Discuss any major changes or questions beforehand in the brackets-dev newsgroup."

Another good way to let the community know what you're working on is to open an issue and then add a comment that you would like to work on it.

[ingorichter]

Thank you to all contributors on getting the native menus working on Linux. I'm sorry to hear, that there was duplicated effort to get this implemented.
I posted a proposal to the forum to resolve this kind of situations.

[JeffryBooher]

@ingorichter what is the state of your branch? This PR is getting really stale.

[ingorichter]

@JeffryBooher let me check this PR. My branch had some merge issues with master and I had no time this week to resolve them. Since we released Brackets, I will have a look again and get the conflicts resolved.

[ingorichter]

@MattSturgeon Thank you. I'm going to update my branch and incorporate your changes there too.

[MattSturgeon]

@ingorichter no problem, I did very little real work here, thanks for getting it merged and working on this.