Add batch proxy menu to Project Explorer and persist Use Proxy state

The Project Explorer previously only had an old Pre-Cache submenu that
required choosing a sequence. Replace it with the same Proxy menu that
the timeline uses:

- Generate Proxy: batch-generates proxies for all selected footage items
  that have an enabled video stream, using ProxyManager and the current
  proxy settings.
- Use Proxy: checkable toggle that enables/disables proxy playback for the
  selected footage.
- Reveal Proxy / Delete Proxy: consistent with the timeline context menu.

Also make Footage::SaveCustom() write the <proxy> element whenever
proxy_enabled_ is true, even if proxy_path_ is empty, and load it back
in LoadCustom(). This preserves the user's Use Proxy preference across
project saves so it is restored on the next open.

Translation files updated with lupdate; zh_CN and zh_TW translations for
the new Project Explorer strings (and the matching timeline strings) are
filled in.

app/node/project/footage/footage.cpp

@@ -553,6 +553,8 @@ bool Footage::LoadCustom(QXmlStreamReader *reader, SerializedData *data)
 			const QString path = reader->readElementText();
 			if (!path.isEmpty()) {
 				SetProxy(path, state, stream, preset_version, enabled);
+			} else if (enabled) {
+				set_proxy_enabled(true);
 			}
 		} else if (reader->name() == QStringLiteral("sourcestarttime")) {
 			QString source;
@@ -593,7 +595,7 @@ void Footage::SaveCustom(QXmlStreamWriter *writer) const
 	writer->writeTextElement(QStringLiteral("timestamp"),
 							 QString::number(this->timestamp()));
 
-	if (!proxy_path_.isEmpty()) {
+	if (!proxy_path_.isEmpty() || proxy_enabled_) {
 		writer->writeStartElement(QStringLiteral("proxy"));
 		writer->writeAttribute(QStringLiteral("enabled"),
 							   proxy_enabled_ ? QStringLiteral("1")