Enhancement: Add support for DEL key and ability to temporarily show a hidden cursor by holding down a key

[tonedef71]

I was not sure if this belonged here or in the Quark VDP project.

The backspace key works great for deleting the character immediately to the left of the cursor. It would also be very useful to be able to delete the character immediately to the right of the cursor by typing the DEL key.

IDEA: I don't know if this would be a hassle to implement, but it would be useful to be able to reveal on-demand the location of an invisible cursor (i.e. a cursor that has been hidden via the VDU 23,1,0 operation). Perhaps it could temporarily be revealed as a flashing (or solid) cursor while a particular key (e.g. the left Alt key) is being held down.