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/core/commandview.lua

@@ -88,6 +88,10 @@ function CommandView:get_scrollable_size()
   return 0
 end
 
+function CommandView:get_h_scrollable_size()
+  return 0
+end
+
 
 function CommandView:scroll_to_make_visible()
   -- no-op function to disable this functionality