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

@@ -1478,16 +1478,18 @@ 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
-    local filename, mx, my = ...
-    local info = system.get_file_info(filename)
-    if info and info.type == "dir" then
-      system.exec(string.format("%q %q", EXEFILE, filename))
-    else
-      local ok, doc = core.try(core.open_doc, filename)
-      if ok then
-        local node = core.root_view.root_node:get_child_overlapping_point(mx, my)
-        node:set_active_view(node.active_view)
-        core.root_view:open_doc(doc)
+    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
+        system.exec(string.format("%q %q", EXEFILE, filename))
+      else
+        local ok, doc = core.try(core.open_doc, filename)
+        if ok then
+          local node = core.root_view.root_node:get_child_overlapping_point(mx, my)
+          node:set_active_view(node.active_view)
+          core.root_view:open_doc(doc)
+        end
       end
     end
   elseif type == "focuslost" then