mobile: Add Signature Viewing UI

Move a few files from part/ to gui/ since they are also used by the mobile/ frontend

So there's 4 things added

the "This document is signed" (and it's variations) banner

The Signature tree on the sidebar/drawer

The Signature properties dialog

And the Certificate properties dialog

The last two are not super great looking but there's not much more you can do when you have to dump lots of data to the user.