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/dialog/configbase/configdialogbase.cpp

@@ -91,4 +91,11 @@ void ConfigDialogBase::AddTab(ConfigDialogBaseTab *tab, const QString &title)
 	tabs_.append(tab);
 }
 
+void ConfigDialogBase::SetCurrentTab(int index)
+{
+	if (index >= 0 && index < list_widget_->count()) {
+		list_widget_->setCurrentRow(index);
+	}
+}
+
 }