cmake: Add support for generating .qrc files

cmake: Add support for generating .qrc files

Most .qrc files we use just map the (partial) contents of a
directory, so that can be autogenerated. Provide CMake functions
that support this endeavour to remove the need of regenerating
resource files if directory contents changes.

Since RCC is stupid, some hacks are required to make this work,
so we can't use either autorcc nor qt5_add_resource(); we must
invoke RCC manually. Clever use of targets and custom commands
as well as external CMake script invocation ensures that the .qrc
files are always up to date even if files change, while avoiding
useless rebuilds (as is currently still the case for i18n).