CMake: Add the INSTALL_DIR argument to gammaray_add_plugin

[Kentzo]

Use this new feature to define the gammaray_geopositioninfosource plugin via the gammaray_add_plugin rather than custom add_library.

[Kentzo]

Failed tests will get resolved after https://github.com/KDAB/GammaRay/pull/940 is merged.

[dfaure-kdab]

#940 was closed, so what should happen to this? Surely it can't go in with failing tests...