The old notation (for example `CtrlG`) causes confusion when combined
with new notation needed for alt (`Alt-g`) due to Alt being case
sensitive. Previously both formats were supported, but the documentation
and defaults used a combination. This commit only uses the new notation
for consistency.
Ref #1470
HandleShellCommand will now return the stdout as a string and
it also takes an additional flag indicating whether it should
wait before closing the shell and returning to the editor.
