terminal: free xkb state on keyboard destruction

Fix leaking the xkb-state during keyboard destruction, leaking lots of xkb
references into the wild.
terminal: free sysview-device names on destruction

Don't leak the device-names during device destruction in sysview. Somehow,
the device-name is "const char*", so make it "char*" first to avoid
warnings when calling free() on it.
bus: fix use-after-free in slot-release

We must not access slot->floating after we possible dropped the last
reference to it. Fix all callback-invocations to first check
slot->floating and possible disconnect the slot, then release the last
reference.