[Feature request] Only allow one instance of QTerminal dropdown per session

[isf63]

Is your feature request related to a problem? Please describe.
It is currently possible to run multiple instances of QTerminal dropdown on the same session. This leads to unanticipated behavior.

Describe the solution you'd like
qterminal --drop does nothing when already running a dropdown instance, per X11/Wayland session. If none is running, it works as existing.

Describe eventual alternatives you've considered
Same, but show/focus the dropdown when an instance is already running.

Context
Extra instances of qterminal --drop in the same session are inaccessible, and only able to be exited via ending the PID.

EDIT: Other relevant flags to handle are --execute, --profile, --workdir. Those should only apply to a new instance.

[yan12125]

There are also discussions about single-instance mode for the dropdown terminal at #1028. Probably two birds can be killed with one stone.

[tsujan]

Done in #1131.