[quickshare] Port configuration window to QQC2 and Kirigami.FormLayout

Summary: This patch ports quickshare's configuration window to QQC2 and Kirigami.FormLayout, as well as matches the General category's icon with the applet's icon.

Test Plan:
Before:

After:

Reviewers: VDG, Plasma, apol, ngraham

Reviewed By: VDG, apol, ngraham

Subscribers: abetts, apol, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D19664