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/core.h

@@ -350,7 +350,7 @@ public slots:
 	/**
    * @brief Show Preferences dialog
    */
-	void DialogPreferencesShow();
+	void DialogPreferencesShow(int start_tab = 0);
 
 	/**
    * @brief Show Project Properties dialog