Add application tray functionality

[RxmTaco]

This adds a new dependency tray_manager and adds basic tray icon functionality (open/close application) selectable from the tray icon menu. The Minimize on close setting's behavior has been changed to hide the application and retain the tray icon, instead of simply minimizing the application. With the previous implementation of minimize on close, there was no option left to exit the application, so this adds a way to exit the application from the tray icon menu without having to kill it with an external task manager. Process signal handlers for SIGINT and SIGTERM have been added, which fixes #842

[SpookySkeletons]

Does this add a launch flag for background start of commet?
Does the tray icon indicate notifications?

[RxmTaco]

The current window management implementation already has the option to pass a command line argument --minimize which remains fully functional. Exception being that instead of just minimizing the application, it now gets minimized to tray during launch.

I had however completely forgotten about notifications, so I have made a new commit which adds a red badge to the tray icon when there are notifications present.

Tray_Notification.mp4

This implementation renders a copy of the default application icon with the added badge overlaid on the image. This image then gets cached to the file cache so it is not re-created every time the icon needs to be updated.
I would like to get some feedback from someone who is more familiar with the caching system if this approach is reasonable and if the caching has been implemented in a desired way.

[RxmTaco]

Additional linux build requirements:
https://github.com/leanflutter/tray_manager#linux-requirements