feat: allow clicking timeline chapter indicators

I don't like this solution, but anything cleaner would probably require converting chapter indicators into individual elements and than managing/updating their positions and sizes in relation to growing timeline, which seems like a lot more work and code, so this'll have to suffice.

closes #370

scripts/uosc_shared/elements/Element.lua

@@ -129,8 +129,9 @@ end
 
 ---@param name string
 function Element:trigger(name, ...)
-	self:maybe('on_' .. name, ...)
+	local result = self:maybe('on_' .. name, ...)
 	request_render()
+	return result
 end
 
 -- Briefly flashes the element for `options.flash_duration` milliseconds.