NEW: add maps, actions and bindings via '+' button (ISX-1040)

[ritamerkl]

Description

Added '+' button on top of Map-View for adding new action maps. Also added a '+' button on top of the action view to add new action items. Added a '+' button for each action element to add new bindings.
Implemented UI and fuctionality.
Link to the ticket: ISX-1040

Changes made

Added: add maps, actions and bindings via the '+' buttons on the UI

Notes

Please write down any additional notes, remove the section if not applicable.

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[lyndon-unity]

Looks like reasonable changes to implement the add functionality
Minor feedback on variable naming but I'm happy for that to land in follow up PR or essentially pre approve

[jamesmcgill]

Looks great @ritamerkl

In addition to @lyndon-unity's comments please rename the PR to something like:
NEW: add maps, actions and bindings via '+' button (ISX-1040)

Note the uppercase prefix and the Jira ticket at the end in parenthesis.
This will be used as the final commit message after squashing.