Redesign intro page

Summary:
Hello guys,

the intro page of the system settings bothered me because it looks kind of outdated.
So I've desided to redesign it. Because this is my first contribution containing a little bit more than just "adding icons", there will be probably some issues with my code / design

Old:
{F7360715}

New:
{F7360717}

Dark Theme:
{F7360718}

Other Language:
{F7360720}

Update "less rounded corners":
{F7361923}

Update "with plasma logo and without light rectangle":
{F7364198}

Test Plan: Open systemsettings and see

Reviewers: #vdg, GB_2, ngraham

Reviewed By: #vdg, GB_2, ngraham

Subscribers: filipf, anthonyfieroni, davidedmundson, mart, Codezela, plasma-devel

Tags: #plasma, #vdg

Differential Revision: https://phabricator.kde.org/D23984