Initial work on dynamic cache cleaning

[dweymouth]

Description:

Taking inspiration from the Go garbage collector, this PR will seek to clean caches whenever they double from their last post-clean size, as well as on the regular clean schedule (currently 1 minute)

Fixes #5486, possibly #3842 and #3499

Checklist:

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

Where applicable:

• Public APIs match existing style and have Since: line.
• Any breaking changes have a deprecation path or have been discussed.
• Check for binary size increases when importing new modules.

[coveralls]

coverage: 62.315% (-0.03%) from 62.34%
when pulling 2a80bd3 on dweymouth:dynamic-cache-clean
into 61d6cb3 on fyne-io:develop.