Update some icon names
This updates some icon names to use the FreeDesktop.org icon naming spec.
This also updates some icons to prefer symbolic icon names for some "problem icons" like Close and Clear. This should make them easier to see when using either light or dark themes.
If the switch to symbolic icons is undesired or should be another MR, let me know and I can switch it.
If those are okay, I believe the only remaining button icons that aren't symbolic are the button icons for the "Are you sure you want to quit?" dialog when using the tray plugin (Minimize, Cancel, Quit). I updated those to FD.org icon names, but not to symbolic names. If it's preferred to have those be symbolic as well let me know and I can update this MR.