Timer: Add connect()/disconnect().

[Mugen87]

Related issue: #30522 (comment)

Description

The PR implements connect()/disconnect() to make the ctor side-effect free.

This change must be noted in the migration guide since calling timer.connect( document ); is now mandatory if the Page Visibility API should be used to avoid large time delta values.

[sunag]

Could this be connected to the Renderer? Maybe this helps to synchronize the time and deltaTime node with the Timer.

[Mugen87]

Maybe something like:

renderer.setTimer( timer );

In this way, the renderer could call timer.update() internally.

[sunag]

I think one timer should come as default in the Renderer, so it can use renderer.getTimer() in the examples if you prefer. Synchronization will help to pause the game for example and preserve the shader's momentum, currently this is only possible if the user creates an isolated timer uniform