Optionally use symbolic icons in the sidepane

Can be enabled with a boolean xfconf value: /misc-use-symbolic-icons-in-sidepane

Looks like this (tango-icon-theme):

Successor of !32 (closed)

Edit: Draft, since it only changes icons for the shortcuts view sidepane … if the setting is called like this, it as well should change icons for the tree-view sidepane