Change clear button from logo to label

Fixes: #336 (closed)

Currently Gnome calculator uses "edit-clear-symbolic" icon to represent the clear button.

This can be misleading considering most calculators either use AC or C for the purpose.

The current implementation makes the user believe that the button is just a backspace meanwhile it is a clear button.

This MR replaces the said button from using logo to using label called 'C'

After change Screenshot

Before Screenshot