KLibrary: what is it good for?

[Alex Merry]

While trying to clean up KDE 4 references in KService, I've been looking
at KLibrary.

This basically does two things: it has a factory() method that has been
deprecated since before kdelibs 4.0.0, and it does some extra lookup magic.

KLibrary::factory() is used by KPluginLoader for compatibility magic
(this has been compatiblity code since 4.0.0, I think, and has a KDE5:
remove comment).

As far as I can tell, the library lookup magic was originally to look
for plugins in a module directory specified by KComponentData.  Now that
this has been removed, it appears to just messily duplicate QLibrary's
own internal lookup logic, except that it looks in "kf5" subdirectories
of QCoreApplication::libraryPaths() (and prints a -- misleadingly worded
-- warning if a library starting with "libkdeinit5_" was not in this kf5
subdirectory).

With this in mind, I would suggest ditching the (pre-KDE-4) compatiblity
stuff and putting KLibrary into kde4support as essentially a typedef for
QLibrary.  KPluginLoader might still want to check the kf5 subdirectory,
since that's the value of ${PLUGIN_INSTALL_DIR} in KDEInstallDirs.cmake.

Alternatively, we could keep KLibrary where it is, but remove everything
except for the lookup magic (which probably wants cleaning up).

Alex