applets/icontasklist: Enable matching through desktop files

See documentation in commit on notes regarding inefficiency. This will be
fixed before v8 is released. Also note we moved away from full paths to
using the "desktop_id", i.e. "google-chrome.desktop".
Due to nonsensical behaviour by certain ... ok sod it. Due to Google Chrome
shipping a package that identifies itself as "google-chrome" yet sets a
WM CLASS of "google-chrome-stable", we need to hold a mapping. So yes,
its your fault Chrome.