Make HoloToolkit feature folders more modular

[NeerajW]

Scenarios:

1. Gina is a developer. She wants to add a script related to Cursors. She should be able to add the script file, prefabs, tests, related materials etc. all into the Assets\HoloToolkit\Input folder aka feature folder. This makes the folder more compartmentalized while contributing.
2. David is a designer and he only wants to keep specific feature folders from the HoloToolkit. The folder structure should be modular enough so users can delete not needed ones easily.

Proposed folder structure:

Assets\HoloToolkit -------------------------> Input ----------------------------------> Input \ Scripts ----------------------------------> Input \ Materials ----------------------------------> Input \ Prefabs ----------------------------------> Input \ Tests -------------------------> Sharing ----------------------------------> Sharing \ Scripts ----------------------------------> Sharing \ Editor ----------------------------------> Sharing \ Prefabs ----------------------------------> Sharing \ Plugins ----------------------------------> Sharing \ Tests

Other folders will match a similar layout. Dependencies between folders will be documented.

[NeerajW]

https://github.com/Microsoft/HoloToolkit-Unity/commit/afb09bd663dac4166b8543817fdaed0603185aba

Merged from the refactor branch.