[Gchempaint-main] Minor suggestions

[Francesco Pietra]

Having used gchempaint 0.8.7 for a publication, two minor suggestions
for a future release:

--- Dashed bond (for below the paper plane) with same-size dashes (to
match the heavy bond) (In addition to the existing cuneiforms)

--- Command to change the molecule on display to the enantiomer

I also noticed that inserting atom numbering outside rings for a
complex structure resulted in a crash when trying to place the number
close to the rings. I had to insert the numbers farther away and then
move them close to the rings. I did not try again, so that it may have
been my fault.

Thanks

francesco pietra