redesign GUI and refactor storage
    
      * added Preview GUI
      * moved Config::setConfig() to AvCaster::SetConfig()
      * added AvCaster::ToggleOutput(), AvCaster::TogglePreview(), AvCaster::ToggleScreencap(),
              AvCaster::ToggleCamera(), AvCaster::ToggleText(), AvCaster::ToggleConfig(), AvCaster::TogglePreview()
      * added AvCasterConfig->setCurrentConfig() and MainContent->reloadConfig()
      * refactored config store into list of semi-static presets and one analogous volatile config
      * changed semantics of AvCasterConfig->configStore from root node to volatile GUI config
      * added AvCasterConfig->configRoot and AvCasterConfig->configPresets
      * added AvCasterConfig::validatePreset()
      * added AvCasterConfig->validateRootProperty(), AvCasterConfig->validatePresetProperty(),
              AvCasterConfig->sanitizeRootComboProperty(), AvCasterConfig->sanitizePresetComboProperty() helpers
      * added AvCasterConfig::presetNames() and AvCaster::PresetNames()
      * added AvCasterConfig::storePreset() and AvCaster::StorePreset()
      * added AvCasterConfig::deletePreset() and AvCaster::DeletePreset()
      * added Gstreamer::Configure() (currently noop)