This new member gives access to the current active panel category (i.e.
tab) if supported, and allows to set its value to another defined
category.
While typically add-ons should not force a tab to be the active one,
there are cases where this is a valid and necessary behavior, e.g. in
'Blender App' where an app can add some new tab to the UI and require
them to be active.
Note that there is a ctach here: typically at start time these panel
categories are unknown (enum is empty), since they are validated by
drawing code. So setting them usually needs to be done after initial
startup and drawing of the UI...
Pull Request: https://projects.blender.org/blender/blender/pulls/114070
Bastien Montagne
f84b0a4dd2