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