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.cpp

@@ -484,9 +484,9 @@ void Core::DialogImportShow()
 	}
 }
 
-void Core::DialogPreferencesShow()
+void Core::DialogPreferencesShow(int start_tab)
 {
-	PreferencesDialog pd(main_window_);
+	PreferencesDialog pd(main_window_, start_tab);
 	pd.exec();
 }