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.
