Merge pull request #659 from phofman/next

Some repository cleanup
Added some comments
Removed unnecessary environment setup
Adding new forms for installation of NDK/simulator
Merge pull request #660 from phofman/next

Moved plugin UI to the next level
Added origin path to NdkInfo

To know, where this info come from.
Changed the order of loaded NDKs

To have first loaded NDKs the plugin can modify
Implementing new update-manager screens
Moved NdkInfo sorting etc. to parent class
Created model class to represent an API Level
Loading available NDKs combines them in API-Level
Improved UI and flow of installing new NDKs
Renamed installation UI to better reflect purpose
Fixed error displayed, when cancelled browsing
Redesigned layout on API-Level options page
Merged NDK and simulator download screens
Added 'forget' support in ViewModels
Added 'forget' support on UI
Renamed a handler to better reflect its purpose
Updated UI and stability of API-Level options