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/preferences/preferences.cpp

@@ -38,7 +38,7 @@
 namespace olive
 {
 
-PreferencesDialog::PreferencesDialog(MainWindow *main_window)
+PreferencesDialog::PreferencesDialog(MainWindow *main_window, int start_tab)
 	: ConfigDialogBase(main_window)
 {
 	setWindowTitle(tr("Preferences"));
@@ -49,6 +49,8 @@ PreferencesDialog::PreferencesDialog(MainWindow *main_window)
 	AddTab(new PreferencesDiskTab(), tr("Disk"));
 	AddTab(new PreferencesAudioTab(), tr("Audio"));
 	AddTab(new PreferencesKeyboardTab(main_window), tr("Keyboard"));
+
+	SetCurrentTab(start_tab);
 }
 
 void PreferencesDialog::AcceptEvent()