Add SizeName property to label to control text size

[dweymouth]

Description:

Allows for controlling the text size of a Label widget. Especially useful with selection support landing in 2.6 as the only other way to customize the size of selectable text would be a ThemeOverride container, which is heavyweight and verbose.

This mirrors the API that already exists for Hyperlink sizing.

Fixes #5561

Checklist:

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

Where applicable:

• Public APIs match existing style and have Since: line.

[coveralls]

coverage: 62.311% (-0.09%) from 62.402%
when pulling b394d19 on dweymouth:label-style
into 4cd00b5 on fyne-io:develop.

[andydotxyz]

I think this really needs some tests, make sure the label is scaling appropriately, updates on change and that the selection follows it...