Restore WindowPlugin
PluginLoader: use a macro for convenience
Add OsxPlugin
Cleanup PluginLoader
Add plugin stub for Windows
Add DockPlugin
Move dock alert to Windows & OSX plugins
Hide system tray on Gnome
Gnome: implement WindowPlugin interface
Gnome: X11 helper

The whole Gnome integration is now in a separate plugin. It
can depend on X11 (extras) for now, until Wayland support is
implemented sometime in the future.
Gnome application menu integration by Venemo
Restore context menus over colorized nicks
Fix BSD license headers
Update README.md