Add support for external language switch

Added new signal InputPanel::externalLanguageSwitch and
a property InputPanel::externalLanguageSwitchEnabled for
enabling external language switch.

If the externalLanguageSwitchEnabled is true, the signal is
triggered instead of built-in language popup.

The new language can be selected by setting the
VirtualKeyboardSettings::locale property.

[ChangeLog] Add option to use external language dialog instead of
built-in language popup.

Change-Id: I44f88e6b3e52db4cfbee02bd3b6d7f4be38a9521
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>