This gets a lot of unecessary includes out of Widget.cpp. Doing this
didn't work before, but improvements in the C library and using dynamic
libraries have likely un-broken it :^).
Also, move the registration global object to an anonymous namespace. No
reason it has to be an extern symbol.
Making an AbstractButton exclusive means that we enforce that only one
of the exclusive buttons within the same parent widget can be checked
at a time.
RadioButton was doing exactly the same thing, except in a custom way.
So just remove the custom code and make it exclusive. :^)
Instead of hard-coding 22 in random places, just make the following
widgets have a fixed height of 22 by default: Button, CheckBox,
ColorInput, ComboBox, RadioButton, SpinBox, TextBox.
In the future we can make this relative to the current font size,
but for now at least this centralizes the setting a bit better.
