Add way to extract icon list

This is a rather simple approach to make sure we'll not forget to add icons to the list again.

Every icon name is marked as such using the icn() function. (hope I didn't forget any)

The icon list can be extracted using ./utils/extract-icons.py, and added to the kirigami_package_breeze_icons()t call.

The only downside right now is that icons used on the c++ side are not extracted. This could be workarounded either by adding /*icn(*/ "im-user-online" /*)*/ or by simply mentioning the icon on the qml side in a comment.