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/task/proxy/proxy.cpp

@@ -79,8 +79,8 @@ bool ProxyTask::Run()
 		 << QStringLiteral("-an")
 		 << QStringLiteral("-vf") << scale_filter
 		 << QStringLiteral("-c:v") << QStringLiteral("libx264")
-		 << QStringLiteral("-preset") << QStringLiteral("veryfast")
-		 << QStringLiteral("-crf") << QStringLiteral("23")
+		 << QStringLiteral("-preset") << params_.preset
+		 << QStringLiteral("-crf") << QString::number(params_.crf)
 		 << QStringLiteral("-pix_fmt") << QStringLiteral("yuv420p")
 		 << QStringLiteral("-movflags") << QStringLiteral("+faststart")
 		 << QStringLiteral("-f") << container_format