Prepend to XDG_DATA_DIRS if it's already defined

If XDG_DATA_DIRS is already defined we need to prepend our path to make
sure we always have it.
Set Qt::X11BypassWindowManagerHint flag for shell windows

If this flag is set, the wayland QPA plugin won't create
wl_shell_surface and we'll be able to use our custom protocol.
Don't set surface format

It's been fixed upstream.
DateTime indicator doesn't show seconds

Don't show seconds by default anymore, this will save a lot of Wayland
messages.