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/init.lua

@@ -1136,6 +1136,8 @@ function core.on_event(type, ...)
     end
   elseif type == "mousereleased" then
     core.root_view:on_mouse_released(...)
+  elseif type == "mouseleft" then
+    core.root_view:on_mouse_left()
   elseif type == "mousewheel" then
     if not core.root_view:on_mouse_wheel(...) then
       did_keymap = keymap.on_mouse_wheel(...)