Clean up background processes when changing tutorial in fyne_demo

[Jacalz]

Description:

All animations and tickers were recreated on loading a new page. Animations resulted in just background CPU usage but the tickers can not (until go.mod defaults to Go 1.23) be GC'd before stop so they would tick in the background forever while basically leaking CPU and memory.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 59.099% (-0.04%) from 59.134%
when pulling dd4dbbf on Jacalz:stop-animations-when-switching-tutorial
into 22234be on fyne-io:develop.

[Jacalz]

I should note that this makes life a lot easier when testing #5422. That's the motivation behind this so there is 0% cpu more often :)

[andydotxyz]

It's not amazing having those string IDs re-declared but it seems ok for this fix. I'm still opting to get the revised demo system in for 2.6 which will be able to refactor this further.