The plugin cache should be saved in $XDG_CACHE_HOME, separated from the Qt configuration in ~/.config

On linux and other similar unix systems, the plugin cache is currently saved in ~/.config/Trolltech.conf, which is also the file where Qt configuration is stored. This is wrong according to the XDG Base Directory Specification. The configuration should be separated from the cache and the cache should be stored in $XDG_CACHE_HOME (~/.cache). I understand that this is not easily fixable in the 4.x series, for compatibility reasons, but I think Qt 5 is a good opportunity to fix this.