[Material] Add support for hovered, pressed, and focused text color on Buttons.

[johnsonmh]

Description

This PR adds support for defining the textColor of buttons when the button is in the hovered, focused, or pressed states.

This is useful for preserving the contrast ratio of the text on a button. Often, when buttons are interacted with, the contrast ratio can drop. This makes the text on buttons more difficult to read, and less accessible.

For example, we can make the blue text in this button darker on hover, and even darker on pressed.

Implementation

The implementation comes in three parts.

1. Creating an enum for MaterialState, this includes, hovered, pressed, focused, dragged, disabled, and, error.
2. Creating a MaterialStateColor class. MaterialStateColor has a resolve method that gets the color given a set of states. Or just use MaterialStateColor.resolveWith(...) and pass a callback that will be used to get a color given a set of states.
3. Updating the RawMaterialButton to keep track of a Set<MaterialState>, the using it to get the color for the current state from MaterialStateColor (or just the color itself if the text color is not a MaterialStateColor).

Tests

I added the following tests:

• Contrast ratio tests for FlatButton, OutlineButton, and RaisedButton in the hovered and focused states.
• Tests that the text/icon of FlatButton, OutlineButton, and RaisedButton can be updated depending on the hovered, focused, and pressed states (using MaterialStateColor).
• Tests for buttons in a blue color scheme. Normally in the blue color scheme, the button text is inaccessible in interactive states, but with MaterialStateColor, they can be fixed.
• Tests that verify that passing a MaterialStatColor to a ButtonTheme still allows you to specify the hovered, pressed, and focused text color of a given button.
• Tests that verify that passing a MaterialStateColor to textColor will cause disabledTextColor to be ignored. Instead, textColor in the disabled state will be used.

Checklist

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• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

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• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[zhancheng]

not only the button, but also the listTile needs the hover and press status to change the color

[HansMuller]

MaterialState and MaterialStateColor can stand alone; better to create material_state.dart.

There's a persistent typo that I eventually gave up on: if(foo) should be if (foo).

[willlarche]

SO good. I hope you're very proud.

[HansMuller]

This looks good, most of the feedback is just about only using expectLater where it's needed.

[HansMuller]

LGTM!