Implement input focus for Mac & Windows

[dathinaios]

As per https://github.com/RustAudio/baseview/issues/152. Still not working for VST but that appears to be due to a separate issue (see https://github.com/RustAudio/baseview/issues/169). I tested it in a nih_plug CLAP plugin in Reaper. I had to modify vizia_baseview to call set_input_focus when the alt-click event is triggered. Will also commit those changes there if this is accepted as a solution.