StringUtil: Add IsPrintableCharacter and use it

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.

Source/Core/Common/StringUtil.cpp

@@ -71,7 +71,7 @@ std::string HexDump(const u8* data, size_t size)
       if (row_start + i < size)
       {
         char c = static_cast<char>(data[row_start + i]);
-        out += std::isprint(c, std::locale::classic()) ? c : '.';
+        out += IsPrintableCharacter(c) ? c : '.';
       }
     }
     out += "\n";