Explicitly support environment variables when editing apps' configs

Add explicit support for environment variables in KMenuEdit via a dedicated
text field people can put them in, which will write them into the desktop file
in a functional and standards-compliant manner.

This is a companion change for
frameworks/kio!1025 to keep
kpropertiesdialog and kmenuedit in sync.

BUG: 461315
FIXED-IN: 5.27