Add Interval Shutter Release

[sbstjn]

Based on #50 , I added a feature to have an interval/timer shutter release. Of course, this could be more complex/generic and have options/configurations available, but it does its job.

The internal interval timer in Fujifilm cameras annoys me; having a "custom interval trigger" was the primary reason to look into your project. Thanks for the great baseline! 🎉 Fujifilm cameras also have support for "external timers," but I haven't look into this.

When active, a command to focus and shutter release are sent on a timed base. Of course, time/interval handling in general is hard thing and this maybe the most naive approach 😂

Anyways, I don't know if things like this should be part of furble or stay in my local fork; also with regards to your planning refactoring mentioned in #50 (comment)

Usage

When connected to the camera, there is a second action next to Shutter called Interval.

When active, it takes a picture every ~2 seconds and shows in incremented number on the screen and a button to stop the interval again.

I have not tested this with anything else than the M5 Core2 and Fujifilm X-T5 camera.

[gkoh]

This looks like a good change, thanks!
Adding intervalometer support has been on my todo for ages.

Let's get the M5Unified blah and Core2 support merged and then we'll get this in.

Even if we need to add things (like timer/interval config) it's a good base to start from.

[gkoh]

@sbstjn I updated your PR and merged it. Thank you!