Add support for rotation by fractional degrees

[logiclrd]

It seems that under the hood, everything is using double to represent angles, so the limitation of rotation to integers is entirely a UI issue. This PR updates the UI to use a standard Gtk SpinButton feature to permit two decimal places in the angle input when rotating a layer. This doesn't affect the step size for the + / - buttons. Tested, seems to be working. :-)

[cameronwhite]

Thanks for working on this! The change looks good 👍

There is an error from the code format check - you should be able to just run dotnet format to fix that

[logiclrd]

I have done this. :-) I amended the commit so that it wouldn't clutter the history with a separate commit just to add two spaces.