Basic Material Design implementation

Updates the UI to use Material Design

• Implement NumberButton to provide custom drawing of number buttons as MaterialButtons.
  • Draw the main number at 2/3 button height
  • Draw the "numbers placed" number (optional) offset to the right
  • If showing notes, offset the numbers to show their relative place within a cell
• Implement IconButton to draw icon buttons with larger icons, and use this for the "clear" and "pencil" buttons.
• Update custom themes to implement requirement Material style names
• Draw a frame around the selected cell, instead of changing the background, to make it clearer to see
• Update Gradle and SDK versions to get the newest behaviour

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• I have opened an issue where I discuss the change I wish to make.
• I'm using my own branch in my local copy. Eg, I want to merge myuser/opensudoku:my-new-contribution into develop.
• All commits are signed.
• I agree to release my code and all other changes of this MR under the GNU GPLv3 license.