add timer effect

Hello,

This MR is an attempt to add the "Timer" MTL effect.

I needed it to showcase some performance improvements of a software I maintain. I edited a video where I could show a before/after comparison with timer showing up the improvements.

To give an idea, something along these lines:

compare-sleep

I have seen some interest for this in Kdenlive on the web:

• https://www.reddit.com/r/kdenlive/comments/kkyjau/how_to_change_the_format_of_dynamic_texts/
• https://www.reddit.com/r/kdenlive/comments/i993oh/how_to_do_a_timer_in_kdenlive/
• https://www.reddit.com/r/kdenlive/comments/u86oy1/change_timecode_format_for_dynamic_text_effect/
• https://www.reddit.com/r/kdenlive/comments/waqcho/dynamic_text_minus_milliseconds/

I'm not sure I implemented the "Start", "Duration" and "Offset" parameters properly, especially the "totime" attribute.

I did not implement the "Speed" parameter because I didn't needed it yet and because I'm not sure it should affect the "totime" attributes (which may hint that this isn't the right direction).