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

src/api/system.c

@@ -301,8 +301,15 @@ top:
 
     case SDL_MOUSEWHEEL:
       lua_pushstring(L, "mousewheel");
+#if SDL_VERSION_ATLEAST(2, 0, 18)
+      lua_pushnumber(L, e.wheel.preciseY);
+      // Use -x to keep consistency with vertical scrolling values (e.g. shift+scroll)
+      lua_pushnumber(L, -e.wheel.preciseX);
+#else
       lua_pushinteger(L, e.wheel.y);
-      return 2;
+      lua_pushinteger(L, -e.wheel.x);
+#endif
+      return 3;
 
     default:
       goto top;