Load screenpool at the same time as we connect to screenchanged signals
Summary:
Otherwise we have a gap during load (waiting querying kactivities))
between screen pool being created and us connecting to the screen
changed signals, which in turn are used to update screen pool.
In particular the primary screen can get out of sync between the current
state and the screen pool.
Test Plan: Based on Christopher Feck's research and initial patch
Reviewers: Plasma
Subscribers: mart, rwooninck, fvogt, cfeck, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D3319
CCBUG:372099 CCBUG:371858 CBUG:371991 CCBUG:371819 CCBUG:371734