Improve Dogtail's performance

Currently we run each command in its own script, importing all modules and initiating Dogtail each time, and many commands depend on re-compute previous results. Our .children() method is particularly bad in this respect.

We should instead run all Dogtail commands in a persistent Python session so results can be stored between Dogtail commands.

Feature Branch: test/12059-dogtail-optimizations

Related issues

Blocked by #11887 (closed)

Original created by @anonym on 12059 (Redmine)

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