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/commands/root.lua

@@ -123,5 +123,13 @@ command.add(nil, {
       return true
     end
     return false
+  end,
+  ["root:horizontal-scroll"] = function(delta)
+    local view = (core.root_view.overlapping_node and core.root_view.overlapping_node.active_view) or core.active_view
+    if view and view.scrollable then
+      view.scroll.to.x = view.scroll.to.x + delta * -config.mouse_wheel_scroll
+      return true
+    end
+    return false
   end
 })