Allow disabling supplemental projects with CMake

Here is another convenience PR, as requested by @Hoikas, that allows us to speed up development. It is now possible to disable building the resource.dat, the launcher and the tools by unchecking the *PLASMA_BUILD_** variables in CMake. The solution has only 85 projects if you disable all of them, and furthermore, you don't have to wait for the compiler to rebuild externalResources, if you e.g. hit F5 to start the Debug client.