Add system tray icon with a simple tooltip

[thothonegan]

This pull request adds a system tray icon, allowing it to run in the background while still keeping an eye on the status of the icecream cluster.

UI wise, has the following changes:

• Adds a new menu item in View which allows you to enable/disable the system tray icon.
• When enabled, it shows up in the system tray using the icemon icon. It also provides a tooltip with the current active jobs.
• Clicking the systemtray icon will hide/show the main window.
• Right clicking the system tray icon has a menu allowing you to directly quit.
• Closing the main window with the systemtray enabled brings up a message telling you its still running in the system tray. Quit on the main menu still directly quits.
• The systemtray state is saved and restored along with the other settings.