Windows Register List fixes

make the rightclick work in the registerlist for all displayed registers.
The LO HI and PC registers couldn't be rightclicked previously. The
weird thing is that all the functions it called already incorporated this
function but you couldn't call it via the gui.

Also, if you rightlick a valid row and then rightclick on the leftmost 16
pixels you'd get the popup menu for the old selection (even on invalid y
indices).

Make the "change register value"-dialog start with '0x' because otherwise
values starting with an alphabetical hexadecimal symbol will be parsed
as a symbol instead of a number and yield 'unknown symbol'