proxy: add dedicated Proxy dialog and preferences fields

- New ProxyDialog (Tools > Proxy Settings..., plus 'Proxy Settings...'
  in the project panel and timeline Proxy submenus) unifying global
  proxy settings, per-footage custom presets, generation and deletion
  in one place instead of three scattered entry points; this also fixes
  the Tools menu action opening the wrong preferences tab
- Preferences Disk tab gains an 'include audio in proxies' checkbox and
  an ffmpeg executable path field (blank = auto-detect)
- Add ProxyDialog smoke tests

app/widget/projectexplorer/projectexplorer.cpp

@@ -33,6 +33,7 @@
 #include "common/define.h"
 #include "core.h"
 #include "dialog/footageproperties/footageproperties.h"
+#include "dialog/proxy/proxydialog.h"
 #include "dialog/sequence/sequence.h"
 #include "projectexplorerundo.h"
 #include "codec/proxymanager.h"
@@ -470,6 +471,11 @@ void ProjectExplorer::ShowContextMenu()
 							}));
 			connect(delete_proxy, &QAction::triggered, this,
 					&ProjectExplorer::DeleteProxiesForSelectedFootage);
+
+			QAction *proxy_settings =
+				proxy_menu->addAction(tr("Proxy Settings..."));
+			connect(proxy_settings, &QAction::triggered, this,
+					&ProjectExplorer::ShowProxyDialogForSelectedFootage);
 		}
 
 		Q_UNUSED(all_items_are_footage_or_sequence)
@@ -690,6 +696,12 @@ void ProjectExplorer::DeleteProxiesForSelectedFootage()
 	}
 }
 
+void ProjectExplorer::ShowProxyDialogForSelectedFootage()
+{
+	ProxyDialog d(this, GetSelectedProxyFootage(context_menu_items_));
+	d.exec();
+}
+
 void ProjectExplorer::ViewSelectionChanged()
 {
 	QItemSelectionModel *model = static_cast<QItemSelectionModel *>(sender());