Added few comments.
Added simple provider presets manager. Now it can't modyfy enything.
Also added ContentProviderFactory, to control ContentProviders and
gather information about it (now in ugly way with creating temporary objects).
Renamed Providermanager into PresetManager.
Also changed its base class from QFrame to QWidget.
Fixed some runtime warning messages.
Added buttons to PresetManager and ability to save settings.
Fixed DerpiBooru check for safe mode.
PresetManager is now QDialog.
PresetManager is now callable from TagBrowser.
PresetManager reset button is now work.
Presets list in TagBrowser now updating after changes performed by PresetManager.
Content provider creation is controlled by ContentProviderFactory.
Added default tags option for SafebooruProbider.
ContentProviderFactory controls destruction as well.