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/codec/proxymanager.h

@@ -64,6 +64,8 @@ public:
 		int height = 720;
 		int version = 1;
 		QString extension = QStringLiteral("mp4");
+		int crf = 23;
+		QString preset = QStringLiteral("veryfast");
 	};
 
 	struct Proxy {