windows: Change encoding of Brick driver .inf file to UTF-8

This allows git to see it as a text file and do proper diffs.
Ensure to unregister callbacks before destroying connected Qt slots

Otherwise a callback might call a already destroyed Qt slot resulting
in a segfault.

Instead of forcing all plugins to keep track of thier registered
callbacks move the device creation into PluginBase. This allows the
destory_ui() function to unregister all callbacks at once.