[applets/mediacontroller] Visually refresh media controller plasmoid

Summary:
The media controller has been adjusted visually. Note that this is marked as WIP due to the fact that the system tray displays margins around the applet, which doesn't look too good.

Co-authored-by: Ismael Asensio <isma.af@gmail.com>

Depends on D28089

Test Plan:
Before:
{F8085938}
After:
{F8089034}

Reviewers: #vdg, #plasma, manueljlin, ngraham, broulik!

Reviewed By: #vdg, #plasma, ngraham

Subscribers: kmaterka, iasensio, ndavis, broulik, gvgeo, davidedmundson, ngraham, manueljlin, plasma-devel

Tags: #plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D27160