Big Sur support and compiling with Xcode 12.5.1

[vsaase]

Hi,
these changes allow to compile Horos with Xcode 12.5.1 and run it on Big Sur.

I changed the signing procedure to ad-hoc signing, as I obviously do not have access to your credentials.
I fixed the glitches with the Toolbar, unfortunately this leads to a bit of waste of screen space.
I updated GDCM to the current version to fix the runtime errors that resulted from different versions of CharLS used by Horos directly (2.0.0) and by GDCM (1.x previously).

Maybe someone could test this on Catalina? I only have access to Big Sur atm.

[DD-P]

Thank you very much @vsaase - this compiles and runs for me on 11.5.1. As far as I'm aware this is the only PR submitted that has allowed Horos to compile on MacOS 10.15/11.x with the relevant Xcode versions.

I hope the maintainers are able to have a look at this in the very near future, as it has the potential to get us over one of the hurdles that has held up Horos development. It would be good to see development proceeding again.

If acceptable for merging, I wonder whether it would be ideal to split the PRs into separate fixes for compiling, toolbar fixes etc and credentials. (I have an old open PR (#581) for handling the signing credentials also.)