St: Ensure to update entry hint visibility with IM preedit

Commit 88ac3397 changed StEntry behavior so the text hint would stay visible while focused, as long as the text buffer is empty. However, IMs that use preedit still should count as "started typing", while the text buffer is still officially empty.

To fix this, check on st_entry_update_hint_visibility() that there's indeed no preedit buffer before showing the hint. We can't directly listen to internal preedit buffer changes in ClutterText, so handle preedit buffer updates through the ::cursor-changed signal that will be indirectly emitted.