[TaskManager] Modernize and reorganize configuration settings

Summary:
This patch ports most of the QQC1 elements into QQC2 and makes use of Kirigami.FormLayout.

Due to an overwhelming number of options originally being present, the patch also splits them up
into "Appearance" and "Behavior" categories.

Test Plan:

Reviewers: Plasma, VDG, ngraham, rooty, GB_2, hein

Reviewed By: Plasma, VDG, ngraham, rooty, GB_2, hein

Subscribers: hein, GB_2, rooty, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19280