Main/testcase: beat some sense into the XPCOM sample code, it was meant to be standalone, and actually was anything but separately compilable due to using IPRT. Fixed the template which is used for building the code, so that this will not slip through again, letting it be so unusable for a long time.

Main: phase out the use of the XPCOM glue (wasn't ever used), and rename tstVBoxAPILinux to tstVBoxAPIXPCOM to match the content libs/xpcom: strip down the standalone XPCOM glue code and disable building it as so far no one uses it