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/view.lua

@@ -144,6 +144,12 @@ function View:on_mouse_moved(x, y, dx, dy)
 end
 
 
+function View:on_mouse_left()
+  self.hovered_scrollbar = false
+  self.hovered_scrollbar_track = false
+end
+
+
 function View:on_file_dropped(filename, x, y)
   return false
 end