feat(web): spacebar text controls

Relates to #949.

* Adds optional `displayName` property to keyboard stubs: if present,
  this will override the name on the spacebar.

* Adds optional `spacebarText` member to `keyman.options`. This defaults
  to `SpacebarText.LANGUAGE_KEYBOARD`, and can be one of:

  ```
  SpacebarText.LANGUAGE
  SpacebarText.KEYBOARD
  SpacebarText.LANGUAGE_KEYBOARD
  SpacebarText.BLANK
  ```

* Sets the display pattern for LANGUAGE_KEYBOARD to be
  "language - keyboard" instead of "language (keyboard)", which
  reduces the number of parentheses visible on the spacebar.