Improve header and metadata display

Summary:
- Moved the icon back onto the page, and removed it from the header
- Moved app metadata below the description, to let users focus on the icon, screenshots, and description
- Added blue lines between each section

Looks better on desktop and mobile now--IMHO of course!

This patch does not include any string changes, so it can go into 5.12.

Supports D9976, which changes the source switcher UI to use a combobox instead of links.

Test Plan:
Tested in KDE Neon (please pardon the fact that the before and after screenshots have different window sizes):

Krita, before:
{F5662756}

Krita, after
{F5664686}

Endless Sky, before:
{F5662755}

Endless Sky, after
{F5664687}

Blender, before:
{F5662758}

Blender, after
{F5664688}

Mobile UI, before:
{F5662945}

Mobile UI, after
{F5664689}

Reviewers: apol, abetts, colomar, #discover_software_store, #vdg, andreaska

Reviewed By: apol, colomar, #discover_software_store, andreaska

Subscribers: matheusm, andreaska, januz, romangg, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9974