SNI implementation for tray icons

[scorpion-26]

Implementation for SNI (StatusNotifierItem) d-bus protocol for tray icons.

Note: This is currently very WIP

Things that need to be done until it is ready for merging into master:

• [x] Implementing basic protocol (item, host and watcher)
• [x] Icon drawing
  • [x] Drawing raw pixbufs
  • [x] Properly align icons
• [x] Removing of items
• [x] Icon changing
• [x] Context menus (Quick and dirty implementation via libdbusmenu-gtk, proper styled implementation later)
• [ ] Hide behind a pretty icon slider
• [x] Configuration: Compile time (Depends on submodules), and runtime (disable/enable)
• [ ] Maybe: Properly implement watcher (So other apps can use it)

Full implementation will close https://github.com/scorpion-26/gBar/issues/5

[scorpion-26]

All major features regarding the dbus protocol are now implemented. The styling for the context menu for tray icons is currently very much broken. The window of the menu is always 200x200, which creates nasty blank space when the background is opaque (I left it transparent for now) and can cause clipping for long menu items (e.g. long names of steam games). This is probably a bug in libdbusmenu-gtk and there is no hope trying to fix it. Since context menus work functionally I'll complete and merge this PR and then try to look into ways to implement the context menu myself.