Adds unit tests for menubar.utils.lookup_icon_uncached.
Tests that icons and themes are found in the icon base directories, and
icons are resolved with the correct priority.
Creates directories and symlinks in spec/menubar/{home,usr} as test
data.
* Unescape strings when parsing desktop entries
* Fix unused value warning in menubar/utils.lua
* Move menubar.utils.unescape() tests
* Clean up menubar.utils.unescape() function
* Fix warning for using "_" in a non-local context
* Do not ignore trailing whitespace in menubar.utils.parse_list()