[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:
{F6682551}

After:
{F6682552}

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