Send mouseleft event when the mouse leaves the window (#928)

* Send `mouseleft` event when the mouse leaves the window

* Call `View:on_mouse_left` when the mouse leaves the `View`

Previously `View:on_mouse_left` was called only when the mouse left the
window, and it was called on every visible `View`.

Now it gets also called when the mouse "changes" `View`, and only the
last `View` the mouse was on will receive the event.

data/core/rootview.lua

@@ -256,8 +256,13 @@ function RootView:on_mouse_moved(x, y, dx, dy)
 
   self.root_node:on_mouse_moved(x, y, dx, dy)
 
+  local last_overlapping_node = self.overlapping_node
   self.overlapping_node = self.root_node:get_child_overlapping_point(x, y)
 
+  if last_overlapping_node and last_overlapping_node ~= self.overlapping_node then
+    last_overlapping_node:on_mouse_left()
+  end
+
   local div = self.root_node:get_divider_overlapping_point(x, y)
   local tab_index = self.overlapping_node and self.overlapping_node:get_tab_overlapping_point(x, y)
   if self.overlapping_node and self.overlapping_node:get_scroll_button_index(x, y) then
@@ -272,6 +277,13 @@ function RootView:on_mouse_moved(x, y, dx, dy)
 end
 
 
+function RootView:on_mouse_left()
+  if self.overlapping_node then
+    self.overlapping_node:on_mouse_left()
+  end
+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)