Update input panel on preedit clearing

[lo48576]

In some cases, input panel does not disappear even when it should.
To reproduce:

1. Switch to Hiragana mode
2. Put something to preedit.
   • For example, type k y to put ky to preedit.
3. Switch to another mode.
   • For example, type l (set-input-mode-latin command in hiragana mode) to switch to Latin mode.
4. Now you see the floating window (with ky for example) even though the current input mode is Latin.
   • Even worse, this does not disappear until you fill preedit with another content or change the window focus.

The patch solves this issue by updating input panel (floating window) when the input mode is changed and the preedit is cleared.