Add assemble option to disassembly view

This adds the possibility to assemble single opcodes at runtime, overwriting the existing one at the address. Can be done through the right click option or by pressing Ctrl+A.

It's mostly code copied from my old assembler, slightly adjusted to remove stuff that isn't necessary, so the quality isn't very good. Some opcodes aren't there yet, but you rarely need them and they can be added over time.