icons: Reorganize icon directories

icons: Reorganize icon directories

Move bundled icon themes into 3rdparty/icons for clarity. Move
Quassel-specific icons from icons/extra-icons into icons proper.
Rename breezedark to breeze-dark to match the upstream directory
name. Update corresponding qrc files accordingly.

Note that this breaks icon loading until the code is adapted to
properly support icon themes and fallback mechanisms. This will
be fixed in a follow-up commit. Since then Quassel itself will
no longer use the hicolor icons (which are only intended to contain
the application icons for use by desktop environments), there is
no need to bundle them anymore.

The import_theme.pl script also still requires adaptations that will
be delivered in a later commit.