Add suppport for custom icons in taskbar overlay

[darthmorf]

I wanted to be able to use a custom image icon in the taskbar overlay in addition to the colours displayed in the general windows.

Features include

• New setting to toggle between coloured square and custom icon
• New setting that opens a file selector dialogue to select an image file to use as the taskbar icon overlay

I looked into functionality to update the actual application icon as well (in order to have a greyscale VS icon that could be coloured to match the rest of the application), but I don't think VS extensions are able to do this sadly.

[madskristensen]

This is great. Thank you!

[darthmorf]

This is great. Thank you!

My pleasure!