chore(windows): Keyman Core UI and Settings

* Adds a taskbar icon to highlight to user when Keyman Core library is
  in use, with a crummy core.ico. This visual feedback will be removed
  in late beta.
* Renames the 'use common core' registry setting to match other flags
* Adds a System Setting to make the Keyman Core setting visible in
  System Settings and kmconfig.