Adds symbolic link icons, better icon updating (fixes #46)

This adds a symbolic link emblem to symlink results. It also rechecks/reloads icons when switching themes (currently, when you switch your icon theme the icons don't change).

The new symlink icon method creates a new GdkPixmap that is larger than the icon (22x21px), then composites the file icon and symlink emblem on top of that. This gives the symlink emblem an offset, similar to Thunar.

With the icon fallbacks (image-missing, text/plain, etc), the rechecking of icons when switching icon themes, and better searching of icon name variations, I removed icon caching and overrides. It didn't make and performance difference and wasn't working too well with the symlink method. If this is a problem let me know and I can add them back and rework it.

Edit: Oh, and shameless plug but in the process of working on this I found some missing icons and some icons that needed updating in the elementary-xfce icon theme. I made some updates and did a pull request here.

Thanks!