Refactor scrollbar into its own file (#1124)

* Move scrollbar to its own file
* Don't call `Scrollbar` functions if `View` is not scrollable
* Allow horizontal scrolling in `Scrollbar`
* Add horizontal scrollbar to `View`
* Add `root:horizontal-scroll` command with `shift+wheel` keymap
* Prioritize vertical scrollbar hover
* Don't send mouse movement to vertical scrollbar when dragging horizontal one
* Fix clicking on horizontal scrollbar track
* Implement `start` scrollbar alignment
* Add documentation to `Scrollbar`
* Make `DocView` infinitely scrollable horizontally
* Handle horizontal scroll SDL event

data/plugins/treeview.lua

@@ -195,10 +195,10 @@ end
 
 function TreeView:on_mouse_moved(px, py, ...)
   if not self.visible then return end
-  TreeView.super.on_mouse_moved(self, px, py, ...)
   self.cursor_pos.x = px
   self.cursor_pos.y = py
-  if self.dragging_scrollbar then
+  if TreeView.super.on_mouse_moved(self, px, py, ...) then
+    -- mouse movement handled by the View (scrollbar)
     self.hovered_item = nil
     return
   end