text-input: Add preedit_commit_mode event for mouse click event

IBus update_preedit_string_with_mode feature send the preedit text and
the commit mode to the input method module, use preedit_commit_mode event
to send the commit mode together with the preeedit text.

IBusInputContext specify how input context interact with the keyboard,
but sometimes mouse click also interacts with the input context, too.

For Firefox, if some preedit text is visible with ibus-hangul input method;
after mouse click, the preedit will be committed twice.

To fix this issue, ibus-hangul can send preedit text and commit mode
together; after mouse click, the preedit is handled directly in
input method module of Firefox, and the preedit is committed in
the current text widget.

For detailed analysis, please read the merge request comments.
URL: !30
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