Find theme plugins in secondary library paths

Since !1120 (merged), Kirigami only finds theme plugins in the first-listed library path (QCoreApplication::libraryPaths()[0]) with no fallback to plugins that are installed in other library locations.

This happens because the conditional break at kirigamipluginfactory.cpp:85 actually breaks every time. The factories hashtable will always contain an entry for pluginName at this point because we've just created one (and set it to nullptr) on line 37.

I propose that we check the value stored in the hashtable and only break here if we've actually found a plugin.