Fix two related IBus issues on KDE Plasma Wayland (and similar DEs):
1. IBus not detected without legacy environment variables:
On Wayland, IBus is configured via the DE's Virtual Keyboard setting
and IBus itself recommends unsetting GTK_IM_MODULE / QT_IM_MODULE.
The detection logic only checked these env vars, so it never found
IBus on a properly configured Wayland session.
Fix: When no IM env vars are set, check if ibus-daemon is running
and has an active engine. If another IM (e.g. Fcitx) is explicitly
configured via env vars, respect that and don't override.
2. IBus text injection silently fails on Wayland:
After switching to the Vocalinux IBus engine, the code called
setxkbmap to capture/restore the keyboard layout. On Wayland,
setxkbmap does not reflect the compositor's state and returns
incorrect results (e.g. 'us' when the user has 'de'). Calling
setxkbmap to restore this wrong layout interfered with IBus's
ability to commit text, causing silent injection failures.
Fix: Skip setxkbmap-based XKB layout management entirely on
Wayland sessions. The Wayland compositor manages keyboard layouts
natively, and setxkbmap can only cause harm there.
Fixes#380
