show attribute value in button

[RBasile]

Having the value visible is useful to verify that a monospaced font has the same attribute for every character or when creating a new character to be able to set up them quickly without having to count the squares

the value in the button calculated when the font is extracted and updated when press + or -

It looks like that:

[tchapi]

Hi @RBasile and thanks for your contribution!

Small nits:

• could you expand (in the issue desc) on the rationale of having the (rather technical) value directly in the UI? A PR with no description whatsoever is a bit dry tbh
• would you mind adding a space after the colon for clarity and readability (in all buttons)?

Thanks

[RBasile]

Thanks for the tips ( it's my first pull request ) I made the change, is it ok ?

[tchapi]

All good, thanks!