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
- [DH] icons/oxygen_kde/48x48/status/dialog-information.png