Debugger: Add file prompt hook to UserInterface.

In some circumstances, it may be necessary to ask the user to locate a file
from the lower layers. Adds the corresponding hook to UserInterface, and a
stub implementation for CommandLineUserInterface.

Adds full implementation for GraphicalUserInterface.

src/apps/debugger/user_interface/cli/CommandLineUserInterface.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright 2011-2013, Rene Gollent, [email protected].
+ * Copyright 2011-2014, Rene Gollent, [email protected].
  * Copyright 2012, Ingo Weinhold, [email protected].
  * Distributed under the terms of the MIT License.
  */
@@ -201,6 +201,13 @@ CommandLineUserInterface::SynchronouslyAskUser(const char* title,
 }
 
 
+status_t
+CommandLineUserInterface::SynchronouslyAskUserForFile(entry_ref* _ref)
+{
+	return B_UNSUPPORTED;
+}
+
+
 void
 CommandLineUserInterface::Run()
 {