refactor: cursor events consolidated into one interface (#483)

`mbtn_left` and `wheel` events are now only fired on a `cursor.on_{event}` object, which resets at the beginning of each frame, so elements need to bind these listeners in their render functions. These properties are overwritable which allows elements to take over cursor event handling from their parents if necessary.

scripts/uosc_shared/lib/utils.lua

@@ -93,6 +93,11 @@ function get_point_to_rectangle_proximity(point, rect)
 	return math.sqrt(dx * dx + dy * dy)
 end
 
+-- Call function with args if it exists
+function call_maybe(fn, ...)
+	if type(fn) == 'function' then fn(...) end
+end
+
 -- Extracts the properties used by property expansion of that string.
 ---@param str string
 ---@param res { [string] : boolean } | nil
@@ -553,6 +558,8 @@ function render()
 	if not display.initialized then return end
 	state.render_last_time = mp.get_time()
 
+	cursor.reset_handlers()
+
 	-- Actual rendering
 	local ass = assdraw.ass_new()
 
@@ -566,6 +573,8 @@ function render()
 		end
 	end
 
+	cursor.decide_keybinds()
+
 	-- submit
 	if osd.res_x == display.width and osd.res_y == display.height and osd.data == ass.text then
 		return