FileManager: Use FileOperation to perform drag&drop copy out-of-process

And display the progress of the copy operation in a separate window. :^)

Note that this patch only updates the drag&drop code path to use the new
mechanism. We still have to go through FileManager and make use of this
everywhere.

We also need to support additional operations, like Move, Delete, etc.

Still, this is quite cool! :^)

Userland/Applications/FileManager/CMakeLists.txt

@@ -1,9 +1,12 @@
 compile_gml(FileManagerWindow.gml FileManagerWindowGML.h file_manager_window_gml)
+compile_gml(FileOperationProgress.gml FileOperationProgressGML.h file_operation_progress_gml)
 
 set(SOURCES
     DesktopWidget.cpp
     DirectoryView.cpp
     FileManagerWindowGML.h
+    FileOperationProgress.gml
+    FileOperationProgressWidget.cpp
     FileUtils.cpp
     main.cpp
     PropertiesWindow.cpp