[shell] Don't resize or reposition panel if we are doing restore

Summary:
PanelView::restore sets various properties including length, thickness,
which would result in panel resize and repositioning. PanelView::restore
also explicitly calls PanelView::positionPanel and ::resizePanel.

This results in positionPanel and resizePanel operation being done
multiple times at startup/adding panel and hence flicker when panel
first shows up. This patch makes resizePanel and positionPanel no-op if
we are restoring panel.

Test Plan: Added panel, restored panel, moved around panel

Reviewers: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D3008