Add a function that safely returns whether a character is printable
i.e. whether 0x20 <= c <= 0x7e is true.
This is done in several places in our codebase and it's easy to run
into undefined behaviour if the C version defined in <cctype>
is used instead of this one, since its behaviour is undefined
if the character is not representable as an unsigned char.
This fixes MemoryViewWidget.
89b0ab2d22