Rework how icons are handled in the build system

Rework how icons are handled in the build system

In preparation of moving to native Qt icon theme support, this
commit reworks the handling of icons in the build system:

* Move all vital icons (i.e. application, channel and nicklist icons)
  into the hicolor theme
* Always embed the hicolor theme, so we always find the vital icons
  as a fallback
* Move the bundled Oxygen theme to oxygen/ and include metadata and
  index files, so QIconLoader can find this theme as a fallback
  (a system-installed theme will be preferred in the future, however)
* Ship all icon sizes rather than just the requested ones, so Qt can
  later pick the most applicable size
* Refactor the Oxygen import script accordingly
* Clean up the CMakeLists.txt
* Adapt resource initialization code