Allow intercepting `filedropped` events

The event is first sent to the underlying `View`; if not handled, it's 
managed as before.

data/core/init.lua

@@ -1478,6 +1478,7 @@ function core.on_event(type, ...)
   elseif type == "minimized" or type == "maximized" or type == "restored" then
     core.window_mode = type == "restored" and "normal" or type
   elseif type == "filedropped" then
+    if not core.root_view:on_file_dropped(...) then
       local filename, mx, my = ...
       local info = system.get_file_info(filename)
       if info and info.type == "dir" then
@@ -1490,6 +1491,7 @@ function core.on_event(type, ...)
           core.root_view:open_doc(doc)
         end
       end
+    end
   elseif type == "focuslost" then
     core.root_view:on_focus_lost(...)
   elseif type == "dirchange" then

data/core/rootview.lua

@@ -272,6 +272,12 @@ function RootView:on_mouse_moved(x, y, dx, dy)
 end
 
 
+function RootView:on_file_dropped(filename, x, y)
+  local node = self.root_node:get_child_overlapping_point(x, y)
+  return node and node.active_view:on_file_dropped(filename, x, y)
+end
+
+
 function RootView:on_mouse_wheel(...)
   local x, y = self.mouse.x, self.mouse.y
   local node = self.root_node:get_child_overlapping_point(x, y)

data/core/view.lua

@@ -98,6 +98,11 @@ function View:on_mouse_moved(x, y, dx, dy)
 end
 
 
+function View:on_file_dropped(filename, x, y)
+  return false
+end
+
+
 function View:on_text_input(text)
   -- no-op
 end