[3.0] Consider removing render cache and make widgets maintain a reference to their renderer

[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

See discussion in #5112 for problems with the current approach of renderers being kept track of separately from widgets. It would be simpler and probably lead to fewer bugs if widgets kept a reference to their renderer and created only one renderer per widget ever, and the renderer just gets GC'ed along with the widget

How to reproduce

n/a

Screenshots

No response

Example code

n/a

Fyne version

2.x.x

Go compiler version

n/a

Operating system and version

n/a

Additional Information

No response