ButtonTextTheme should have onPrimary option

[feinstein]

ButtonTextTheme should have a onPrimary (and etc.) option, just like colorScheme has, this way the text inside a RaisedButton with a primary color and the background, would automatically get the right color for it's text, which is usually white or black, depending on the luminescence of the color, even if the ThemeData.brighness doesn't match the primary color.

Also, adding the option to set the actual color for text inside a button and thus be able to override any other options would be very much welcome.

[HansMuller]

@feinstein We're proposing a rather different approach for customizing the appearance of buttons in #54776. If you have time, we'd appreciate feedback on the proposal.

I'm going to close this issue in favor of #54776 .

[feinstein]

Ok Hans, I will take a look at it next week.

[github-actions]

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