[applets/systemtray] Hide/show expander arrow - regression fix

Summary:
Expander arrow should hide when all icons are visible and no icon is hidden. When dialog with hidden items is hidden ListView does not update its count property - property is update on redraw. As a result expander arrow is still visible/hidden until user clicks on any plasmoid to show dialog.

BUG: 420924

Test Plan:
* select/deselect "Show all items" in settings, save, expander arrow should hide/show imidiatelly
* select almost all items to "Always shown", leave "Keyboard indicator" as "Shown when relevant". Press Casp Lock, expander arrow should hide/show correctly

Reviewers: #plasma_workspaces, #plasma, ngraham, broulik, davidedmundson

Reviewed By: #plasma_workspaces, #plasma, broulik, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28180