Add button to change font size as requested in #12

[zjosua]

Description

Concisely describe what the pull request is trying to achieve. If pertinent, link to an existing issue report, or briefly explain the problem the PR is meant to solve. Feel free to attach screenshots or other media for UI-related changes.

issues/todo:

• credit icon author: Freepik on Flaticon (License); edited by me
• maybe check for hotkey conflicts more thoroughly (I didn't have any issues so far)
• Anki's default font size is between 4 and 5, therefore changing back to the default font requires using the Remove formatting button.
• Values greater than 7 default to font size 7.

Checklist:

Please replace the space inside the brackets with an x and fill out the ellipses if the following items apply:

• [x] I've read and understood the contribution guidelines
• [ ] I've tested my changes against at least one of the following Anki builds:
  • [ ] Latest standard Anki 2.1 binary build [required for Anki-compatible 2.1 add-ons]
  • [ ] Latest alternative Anki 2.1 binary build
  • [x] Running Anki 2.1 from source (commit ed8340a)
  • [ ] Latest Anki 2.0 binary build [required for Anki 2.0-compatible add-ons]
• [x] I've tested my changes on at least one of the following platforms:
  • [x] Linux, version: Debian bullseye
  • [ ] Windows, version:
  • [ ] macOS, version:
• [ ] My changes potentially affect non-desktop platforms, of which I've tested:
  • [ ] AnkiMobile, version:
  • [ ] AnkiDroid, version:
  • [ ] AnkiWeb