Implement input focus for Mac & Windows

[dathinaios]

As per #152. Still not working for VST but that appears to be due to a separate issue (see #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.

[micahrj]

The Windows and Linux builds will fail without definitions of has_input_focus and set_input_focus on their platform::Window types. Please add some stub definitions with unimplemented!() to the Windows and Linux backends.