Fix MemoryViewWidget background colour

[noahpistilli]

In commit 6f4f4e0 on line 552, the background of the memory viewer was filled to be white. In dark mode this would make it impossible to see the contents of the viewer.

[Pokechu22]

This change results in the cell staying red if you right-click and select "toggle breakpoint" twice (or if you scroll when a breakpoint is on screen).

I think using cell->setBackground(Qt::transparent); would give the desired result without causing that problem, though?

[noahpistilli]

Good catch, just tested and using that gives the desired result.