UI: Confirm dialog when closing an unsaved file

The complexity in this patch comes from the fact
that the current operator system does not support
multi-step user interactions well.

More specifically, for this to work, we need to show
a confirm dialog and a file browser afterwards.
We decided that it is easier to keep everything in
a single operator, instead of creating separate
operators that invoke each other.

So, now the `WM_OT_open_mainfile` operator invokes
itself in different states. It implements a simple
finite state machine to manage the states.

The dialog itself is expected to be improved in
a future commit. See D4829 for more details.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D4829

release/scripts/startup/bl_operators/wm.py

@@ -1747,8 +1747,10 @@ class WM_OT_drop_blend_file(Operator):
         layout = menu.layout
 
         col = layout.column()
-        col.operator_context = 'EXEC_DEFAULT'
-        col.operator("wm.open_mainfile", text="Open", icon='FILE_FOLDER').filepath = self.filepath
+        col.operator_context = 'INVOKE_DEFAULT'
+        props = col.operator("wm.open_mainfile", text="Open", icon='FILE_FOLDER')
+        props.filepath = self.filepath
+        props.display_file_selector = False
 
         layout.separator()
         col = layout.column()