[comic] Modernize configuration windows

Summary:
All 3 comic's configuration windows (General, Appearance, Advanced) have been ported to QQC2 and Kirigami.FormLayout. Other changes include:

• the icon for the Appearance category is now consistent this with the icon used for this category elsewhere
• improving the wording of options
• removing the sections in Appearance and Advanced tabs
• making it explicit that the navigation buttons can always be shown or only on hover

Test Plan:
Before (General):

After (General):

Before (Appearance):

After (Appearance):

Before (Advanced):

After (Advanced):

Reviewers: rooty, ngraham, VDG, Plasma

Reviewed By: ngraham, VDG

Subscribers: abetts, mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19802