[desktoppackage] Port to QQC2 and fix "Get New Widgets..." button text
- Port to QQC2
- Fix "Get New Widgets..." button text
Test Plan: Try pressing Esc in the widget explorer dialog or alternatives dialog or in the panel edit mode. Also look at the "Get New Widgets..." button in the widget explorer dialog.
Reviewed By: VDG, ngraham
Maniphest Tasks: T10586
Differential Revision: https://phabricator.kde.org/D20904