Gives immediate focus to the search entry control after pressing the shortcut to show it

Summary:
when showing the filter panel, give it focus immediately

clean up the logic behind the filter bar expanded and collapsed states

CCBUG: 398718

Test Plan: pressing the shortcut gives focus

Reviewers: astippich, januz, ognarb

Reviewed By: astippich

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D15607