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/widget/timelinewidget/timelinewidget.cpp

@@ -1196,6 +1196,10 @@ void TimelineWidget::GenerateProxiesForSelectedClips()
 		}
 
 		ProxyManager::ProxyParams params;
+		params.width = OLIVE_CONFIG("ProxyWidth").value<int>();
+		params.height = OLIVE_CONFIG("ProxyHeight").value<int>();
+		params.crf = OLIVE_CONFIG("ProxyCRF").value<int>();
+		params.preset = OLIVE_CONFIG("ProxyPreset").toString();
 		const ProxyManager::Proxy proxy =
 			ProxyManager::instance()->GetOrStartProxy(
 				item->project()->cache_path(), item->filename(),