Investigate whether we need to implement focus_hint=false

The focus_hint setting, which defaults to true, implements a workaround for buggy X11 clients when it's set to false. Some old, old (really old) X11 clients may set their input hint to false, and not include WM_TAKE_FOCUS in WM_PROTOCOLS (which results in a "None" input model), but they actually need focus.

We can't implement this in xfwl4 without either patching smithay to let us override the input model, or implementing an invasive workaround in xfwl4.

Arguably this workaround isn't needed anymore: anything written with a toolkit today uses the correct behavior. Are there old, bespoke X11 apps that might need this still? Maybe. But let's wait and see before deciding to go to the effort to implement this.