Adds a setting (default false) to use custom tray icon image

[NathanC]

This adds in a setting to use a custom tray icon. It works with svgs, and I've got it set on my system now to use the same icon for ksnip from my icon pack.

New Settings

(note: ksnip doesn't seem to be able to take screenshots of it's own settings window so I had to use Spectacle, not sure if that's intentional or not)

Example custom icon

This is the first time I touched C++ since college, so if I did anything wrong let me know!

TODO:

• Display error message to user on falling back to default icon, or at least log it somehow
• Run tests (if they're not done automatically by GitHub)

[DamirPorobic]

Hi there, sorry for waiting so long for this PR, I was on vacation and recently get almost no time for developing ksnip. I like the idea of having this kind of customization :)

I just had a brief look though the PR, more will hopefully come in next couple of days. Regarding the radio buttons, we usually have them vertically aligned, you have it now horizontally, any specific reason or could you place them vertically to be more inline with the remaining ones?

Unit tests are run by the CI automatically.