Add new feature "Auto commit word"

This feature, when enabled, automatically commits the remaining single
word in the word candidate list, when there was initially multiple
candidates.

This feature can be enabled from settings using
VirtualKeyboardSettings.wordSelectionList.autoCommitWord property.

[ChangeLog] Added support for automatic selection of the only remaining
word in the suggestion list.

Change-Id: Iecfafcaeb3ccb72e1d8e53b92c395b0d1dc84163
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>