Redesign the theme preview window

Summary:
This patch:
- makes the root element accommodate its content's dimensions so that information doesn't get cut off when using scaling or big fonts
- matches the root element's color to that of the surrounding space
- replaces the border around the image preview with a drop shadow
- ports the GridLayout to ColumnLayout to make sure content doesn't get lost horizontally when using scaling or big fonts
- ports Text elements to QQC2 Labels
- adds the theme name as a Kirigami heading
- constrains the labels within the layout's width, wrapping and eliding them if necessary
- removes the hardcoding of font sizes
- makes the email and website info clickable

BUG: 372844
FIXED-IN: 5.16

Test Plan:
Before:
{F6618593}
After:
{F6630795}

Reviewers: #plasma, #vdg, ngraham, GB_2

Reviewed By: #vdg, ngraham, GB_2

Subscribers: mart, GB_2, mmustac, davidedmundson, abetts, rooty, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19077