Better build/install/run instructions for KXmlGui tutorial
This:
- conciliates build instructions into a single source folder for kdesrc-build, raw CMake, and in the future Craft: ~/kde/src/project
- in the CMake case, actually install and run the application from the start using the well established ~/.local path as --prefix
- because of the above, it is no longer necessary to source the prefix file
- switches some mentions from kde-builder to kdesrc-build that were added via a direct commit
- removes unnecessary explanations about CMake
In doing so, the instructions got significantly shorter.