Offer an optional "tray icon"

[Zirias]

Add an option to have Xmoji display a "tray icon", with the possibility to hide Xmoji from the taskbar when the tray icon is shown.

Implementation notes:

• Only implement the old X11 protocol based on XEmbed. We don't want to introduce a dependency on dbus, so desktops only implementing the dbus method won't be supported, this is acceptable.
• The tray icon shall offer to bring Xmoji to the front of the current virtual desktop with a single click.
• A popup menu on the tray icon shall give access to the dialogs (About, Settings) and terminating Xmoji.
• It shall be configurable whether closing the main window will just hide Xmoji or terminate the application when a tray icon is shown.