Create new API for managing a temporary mode.

[wengxt]

Summary

Right now, there are modules, including unicode, quickphrase, input method selector, that are acting as an engine independent mode.

They share lots of common code:

1. A shortcut to trigger the mode
2. grab all keys after mode is triggered.
3. release the grab after existing the mode

We want to introduce a new API, to:

1. reduce the code duplication for such usecase
2. at the same time, additionally return the descriptive name of such mode, as a replacement of "inputMethodLabel" and "inputMethodEntry" for display the current name of the mode.