Memory leak in font metrics cache

[dweymouth]

Checklist

• I have searched the issue tracker for open issues that relate to the same problem, before opening a new one.
• This issue only relates to a single bug. I will open new issues for any other problems.

Describe the bug

The fontSize cache here is only cleaned out on settings change, and it stores arbitrary user-supplied strings in the map key. It should be made an expiring cache and have expired entries cleaned out during the periodic clean task.

How to reproduce

Run fyne_demo and scroll through all rows of the list/table demos, or any other text-heavy Fyne app.

Screenshots

No response

Example code

fyne-demo

Fyne version

develop

Go compiler version

go1.19.4 darwin/amd64

Operating system and version

Mac OS

Additional Information

No response

[andydotxyz]

Thanks @dweymouth for resolving this (on develop) for v2.4.0 :)