Misc. NUI editor fixes

[rzats]

Contains

Some additional minor fixes to the features introduced in #2415:

• Boolean attributes now default to their inverse value when added.
• Fixed an issue with NUIEditorItemRenderer not properly displaying identical node values.
• Fixed an issue with ContextMenuScreen being drawn out of bounds for items below the screen.

How to test

Verify that the fixes work as intended and no other usability bugs remain.

Outstanding before merging

No outstanding issues.

[GooeyHub]

Hooray Jenkins reported success with all tests good!

[flo]

thanks for the pull request. Having the context menu not go outside of the visible area is really nice.

[Cervator]

Cool, does this count as a fix for #1892 then? That sounds very similar but I can't test the exact spot the example is from (discovered #2429 while trying to test it)

[rzats]

@Cervator: not quite, since this changes ContextMenuScreen that didn't exist when #1892 was created (and is currently only used within the editor). Though I can look into fixing that issue after GSOC 😄