Escape reserved characters when converting paths to urls

* Introduce and use BUrl::BUrl(const BPath&)
* The path is url-encoded, and the protocol is set to "file"

Fixes #10964.

headers/os/net/Url.h

@@ -8,6 +8,7 @@
 
 #include <Archivable.h>
 #include <Message.h>
+#include <Path.h>
 #include <String.h>
 
 
@@ -17,6 +18,7 @@ public:
 								BUrl(BMessage* archive);
 								BUrl(const BUrl& other);
 								BUrl(const BUrl& base, const BString& relative);
+								BUrl(const BPath& path);
 								BUrl();
 	virtual						~BUrl();