gui: Enable console line edit on setClientModel

Summary:
It was possible to cause bitcoin-qt to crash by executing any command in menu Window > Console during startup.
This is fixed by disabling console line edit by default, and only enabling once RPCConsole::setClientModel is called.

Backport of Core PR16122

Test Plan:
ninja && ninja check

Note: I wasn't able to reproduce the crash, I'm not able to reach the menu before the application is well started.
But I tested that the console is still usable after this diff.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7803