Should check for keyboard caps before claiming keyboard

While I haven't checked yet why keyboard caps are missing on my Nokia flip here's patch to check for the needed capabilities. Otherwise we fail with: as the server sends a protocol error:

wl_seat#28: error 0: wl_seat.get_keyboard called when no keyboard capability has existed

(bananui-shell:24972): layer-window-CRITICAL **: 05:57:45.611: mmap: Invalid argument

(bananui-shell:24972): bsh-shell-ERROR **: 05:57:45.611: Failed to initialize top panel
Trace/breakpoint trap

Patch is at https://github.com/agx/bananui-shell/commit/e0c052fa6db7823b9878c8c6377c1d03dc49c334 (as I couldn't create a fork)