SearchView: Fix missing signal/slot connection to project plugin

Summary:
...to notice project change after session switch

Without this patch was the project offer in the m_ui.searchPlaceCombo
after a session switch gone

Reviewers: Kate, sars

Reviewed By: Kate, sars

Subscribers: sars, kwrite-devel, Kate

Tags: Kate

Differential Revision: https://phabricator.kde.org/D17296

BUG:347311