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/node.lua

@@ -51,6 +51,15 @@ function Node:on_mouse_released(...)
 end
 
 
+function Node:on_mouse_left()
+  if self.type == "leaf" then
+    self.active_view:on_mouse_left()
+  else
+    self:propagate("on_mouse_left")
+  end
+end
+
+
 function Node:consume(node)
   for k, _ in pairs(self) do self[k] = nil end
   for k, v in pairs(node) do self[k] = v   end
@@ -160,8 +169,12 @@ end
 
 function Node:set_active_view(view)
   assert(self.type == "leaf", "Tried to set active view on non-leaf node")
+  local last_active_view = self.active_view
   self.active_view = view
   core.set_active_view(view)
+  if last_active_view and last_active_view ~= view then
+    last_active_view:on_mouse_left()
+  end
 end