Automatically hide word candidate list

This change adds support for automatically hiding word candidate list
when inactive. This feature includes the following enhancements:

- Added new settings:
  * VirtualKeyboardSettings.wordCandidateList.autoHideDelay
  * VirtualKeyboardSettings.wordCandidateList.alwaysVisible
- Automatic hiding of word candidate list when inactive and when
  autoHideDelay elapsed.
- alwaysVisible setting restores the old functionality.
- Added new signal selectionListsChanged() to input method, allowing
  the input method to dynamically allocate or deallocate selection
  lists.
- HunspellInputMethod does not allocate selection list when
  dictionary cannot be loaded, or Qt::ImhNoPredictiveText is enabled.
  Also, it will no longer use pre-edit text in this case.
- OpenWnnInputMethod does not allocate selection list if not needed.

[ChangeLog] Automatically hide word candidate list when inactive.

Change-Id: Ifa95ae8a7c47a96719ffdc2929601ff2ef9c0d2e
Reviewed-by: Gordan Markus <gordan.markus@pelagicore.com>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>