Improve header and screenshots

Summary:
Taking inspiration from @abetts [[ https://www.youtube.com/watch?v=QfiHLfrob2A&feature=youtu.behttps://www.youtube.com/watch?v=QfiHLfrob2A&feature=youtu.be | popular idea for some tweaks ]], I implemented a few changes to the Application page:
- Increased size of app icon, name, and caption
- Removed blue line separating header and screenshots/description, since it was controversial and kind of pointless and inconsistent when the metadata section didn't have a text header
- Displayed screenshot thumbnails with the aspect ratio of the full-sized image, so they look better and nothing is cut off in the thumbnail
- Gave screenshot thumbnails a drop shadow and more padding between them

Errata:
- The screenshots lost their hover effect as a side effect of the way the QML DropShadow effect had to be implemented. I tried many ways of adding it, and this was the way that caused the last amount of damage. Definitely open to suggestions.

Test Plan:
Tested in KDE Neon:

Pitivi, before:
{F5666174}

Pitivi, after:
{F5666175}

Audacity, before:
{F5666176}

Audacity, after:
{F5666177}

Clementine, before:
{F5666178}

Clementine, after:
{F5666179}

Mobile view Kdenlive, before:
{F5666198}

Mobile view Kdenlive, after:
{F5666199}

Mobile view Endless Sky, before:
{F5666201}

Mobile view Endless Sky, after:
{F5666202}

Mobile View gedit, before:
{F5666209}

Mobile View gedit, after:
{F5666210}

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

Reviewed By: apol, abetts, #discover_software_store, #vdg

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10002