Add back QInputMethodEvent support and tests to Widget API

The input method event support is partly implemented but not tested properly in Qt WebEngine. Try to revive the Qt WebKit behavior, move the input method support form QWebEnginePage to QWebEngineView, and re-enable corresponding widget test cases.

Currently the following issues are known:

• IME doesn't support colored underline
• IME background support might be added
• Text selection is handled via IME which is not handled as selection in Chromium -> unable to query selection via QInputMethodQuery
• Text selection cannot been cancelled