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/keymap-macos.lua

@@ -34,7 +34,8 @@ local function keymap_macos(keymap)
     ["cmd+8"] = "root:switch-to-tab-8",
     ["cmd+9"] = "root:switch-to-tab-9",
     ["wheel"] = "root:scroll",
-    
+    ["shift+wheel"] = "root:horizontal-scroll",
+
     ["cmd+f"] = "find-replace:find",
     ["cmd+r"] = "find-replace:replace",
     ["f3"] = "find-replace:repeat-find",