Restore ability to export single data item to specific filename

[jamesrussell216]

Original changes where made to bring the single item export into line with the multi-file export as per this issue: #932

However this has introduced more steps into doing a single item export. Probably where we went wrong was that issue was asking to be able to have the same export options for a single export as a batch, not forcing them to have the same.

From @cmeyer ...

The main feature I want is to be able to export and name my file. Doing that for a single data item is not an obvious experience and leaves the export dialog in a weird state: uncheck all of the boxes, and enter your filename in the "prefix".

I propose three menu items (for now):

Export Item... (works on a single item, uses title of item, displays error message if fails)
Export Items... (works on multiple items, uses dialog for naming, displays log of success/failures [always])
Export SVG... (unchanged, for now)

2nd item could also be "Batch Export..."

Is it perfect? Probably not. Does it accomplish what we want? Probably. Let's do it and move on.

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[jamesrussell216]

Some minor suggests UI-wise just to hopefully make things a bit clearer...

For the file menu and right-click context menus (so same functionality for both)

• A single "Export..." menu item which expands to the three sub menu items suggested above...
  - "Single Item..."
  - File save menu dialog, default name as item title. Displays error message if fails.
  - "Batch Export..."
  - New multi-export window
  - "Export SVG..."
  - Existing SVG panel

Behaviour...

If a user has only one item selected, allow all three sub menu items (so that you can still access the batch export windows for a single item export, as originally requested in #932 but also do a quick export if desired)

If a user has multiple items selected, disable the "Single Item..." option as this doesn't make sense in this context.

[cmeyer]

This sounds good!

[cmeyer]

Fix applied 3e084a5

[cmeyer]

@jamesrussell216 can we close this?

[jamesrussell216]

Yep