[desktoppackage] Port to QQC2 and fix "Get New Widgets..." button text

Summary:
* 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.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel, #vdg, #plasma

Tags: #plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D20904