Fix plugin load failures

[Dmitry A. Kuminov]

Qt core caches plugin information once it tries to load a Qt plugin. If, at this time, the plugin cannot be loaded (i.e. due to missing dependencies among DLLs), an entry saying that the plugin is invalid will be stored in the cache (and taken from there next time the plugin is loaded). This cache is not updated until the plugin DLL timestamp changes (or the cache is manually cleared). As a result, even if later all DLL dependencies are satisfied, Qt will still refuse to use it giving a message like "The file XXX.DLL is not a valid Qt plugin."

This is completely wrong and needs to be fixed. It's a generic Qt problem.