Added symbols icon and fraction key to second page

[tomkuschka]

Description

This PR adds a new icon for the symbols page and a fraction button to the second page of the keyboard

New icon for symbols	Fraction button

Checklist

Remove If [...] items that do not apply to your PR.

• [x] I have made myself familiar with the CaTeX contributing guide.
• [x] I added a PR description.
• [x] If this PR changes anything about the main math_keyboard or example package (also README etc.), I created an entry in CHANGELOG.md (## UPCOMING RELEASE if the change on its own is not worth an update).
• [x] If this PR includes a notable change in the math_keyboard package, I updated the version according to Dart's semantic versioning.
• [ ] All required checks pass.

[creativecreatorormaybenot]

(but would be cool if you replace the accent color usage by color scheme secondary :D)