Determine an identifier for the ABI of the probe we are currently building.
Add GAMMARAY_PROBE_ONLY_BUILD option.

This allows to only build the target-specific subset, ie. the part you can
have installed multiple times (e.g. for Qt4 and Qt5).

All these files still need to be appropriately namespaced of course, to
make this actually work.
Don't hardcode the library installation directory, fails with lib64.
Change the install location for our plug-ins.

So far we put GammaRay plug-ins into the Qt plug-in directory (which is
specific to one Qt version) and the other GammaRay dynamically loaded
modules such as the probe directly into lib/ (which is rather dirty, to
say the least). Now it all ends up in a GammaRay sub-folder under lib/
(on UNIX, plugins/ parallel to bin/ on Windows), with sub-folders for
GammaRay version and probe ABI.
The style injector still needs to go into the Qt plug-in dir of course.
Put the plug-ins in the same place as the probe.
Add the probe ABI also to the DLL names.

Now we do not install a single file without the probe ABI included in some
form when doing a probe-only build.

This should enable co-installability of a Qt4 and Qt5 probe, just the
ability to select the right one in the injector is missing.
Add a command line option to list all installed probes.
Command line interface for selecting a specific probe ABI.

We'll use that until we have full probe ABI autodetection.
Actually this already works the way it is.

This is library code that works exactly for one ABI, so having the ABI
identifier compiled in here is just fine.
Properly handle Qt5 version numbers here.
Allow to select the probe ABI also in the UI.
Remember settings from the attach page too.
Pass on the probe ABI from the launcher UI correctly.