St: Ensure to update entry hint visibility with IM preedit

St: Ensure to update entry hint visibility with IM preedit

Commit 88ac339774 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.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1084