Implement showConfigurationInterface for widgets in scripting

Remilia S.requested to merge
remilia-dev/plasma-workspace:widget-config-interface into master

This change is based on the code in containment.cpp.

Reason for the change

I have a plasmashell script where it'd be nice to be able to open the configuration interface of widget (so that it can then be screenshot to compare with previous test runs).

Test plan

I've tested it locally. Widgets with a configure action open up a configuration window while those that don't have a configuration action do nothing.

It's my first time trying to commit something to KDE so hopefully I've done this right. There is no bug report in the tracker for this but I can create one if you'd like to have a bug report attached.

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