Support using system icons with semantics (Linux)

[TacoCake]

What?

Right now, we're always converting to an absolute path.

This means that if you're trying to create a notification with the system's dialog-warning icon, the notification's icon will end-up blank / missing.

Why?

Instead of adding extra logic to figure out the absolute path of system icons, I think we should just be able to use the semantic name.

Other possible solutions

If you don't like adding this check to the pathAbs function, I could simply move the check to the notify_unix.go:17 file directly, as to not impact any other platforms.

Calling a notification with the icon dialog-warning

Before PR:

After PR:

[!NOTE] I have not tested any other platforms, only linux with KDE plasma