The ForwardKeyEvent ibus signal is used to maintain ordering between CommitText events and non-IME key events.
The ordering is important for Korean IMEs, where it automatically commits text whenever a character is fully composed. This is different from Chinese/Japanese IMEs, where user manually presses a key to commit each word.
Without this patch, kitty can't handle non-IME key events when used with ibus-hangul, which utilizes ForwardKeyEvent signal.
Without ForwardKeyEvent:
```
key | sequence of events from application's view
r | UpdatePreeditText 'ㄱ'
k | UpdatePreeditText '가'
1 | (receives the reply to ProcessKeyEvent call: "native_key: 0x31 release: 0 handled: 0")
| -> UpdatePreeditText ''
| -> CommitText '가'
```
Application receives "1가", which is incorrect.
With ForwardKeyEvent:
```
key | sequence of events from application's view
r | UpdatePreeditText 'ㄱ'
k | UpdatePreeditText '가'
1 | (receives the reply to ProcessKeyEvent call: "native_key: 0x31 release: 0 handled: 1", and kitty discards the event)
| -> UpdatePreeditText ''
| -> CommitText '가'
| -> ForwardKeyEvent keysym=0x31("1")
```
Application receives "가1", which is correct.
Relevant ibus-hangul github issue to implement ForwardKeyEvent: https://github.com/libhangul/ibus-hangul/issues/42
Relevant changes in Qt to handle ForwardKeyEvent: https://codereview.qt-project.org/c/qt/qtbase/+/212179, https://codereview.qt-project.org/c/qt/qtbase/+/255587
fcitx5 works fine with kitty, except that when you restart fcitx5, kitty stop working with fcitx5, because the dbus_connection_get_is_connected(ibus->conn) in #check_connection is always true when you use fcitx5.
this patch add a dbus name owner changed detection to detect the online status of ibus, in case the ibus backend uses the session bus, like fcitx5.
This is required when the actual dbus is not from ibus but session bus.
This may happen when other run a shim layer for ibus, or ibus is
running with flatpak mode.
