Add configurable proxy settings and a menu entry

- Extend ProxyManager::ProxyParams with crf and preset, and update
  ProxyTask/ProxyParamsEqual to use them.
- Add global config keys ProxyWidth, ProxyHeight, ProxyCRF, ProxyPreset.
- Add a Proxy Settings group to Preferences -> Disk with width/height/CRF/preset
  controls.
- Read proxy settings from config when generating proxies from the timeline.
- Add Tools -> Proxy Settings... menu entry that opens Preferences on the Disk tab.
- Add ConfigDialogBase::SetCurrentTab() so PreferencesDialog can start on a
  specific tab.

app/window/mainwindow/mainmenu.h

@@ -281,6 +281,7 @@ private:
 	QAction *tools_add_item_;
 	QAction *tools_record_item_;
 	QAction *tools_snapping_item_;
+	QAction *tools_proxy_settings_item_;
 	QAction *tools_preferences_item_;
 	Menu *tools_add_item_menu_;