Make KMessageWidget match Kirigami inlineMessage's style

Summary:
This patch makes the venerable and wonderful KMessageWidget match the slick style of Kirigami's new inlineMessage control. As a by-product, it fixes a few usability bugs.

I didn't implement the drop shadow here because I couldn't get it to look right.

BUG: 385299
BUG: 381255

Test Plan:
Spectacle, Breeze light, information:

Spectacle, Breeze dark, information:

Spectacle, Breeze light, positive:

Spectacle, Breeze dark, positive:

Dolphin, Breeze light, negative:

Dolphin, Breeze dark, negative:

Dolphin, Breeze light, warning:

Konsole, Breeze light with dark profile theme, warning:

Konsole, Breeze dark with light profile theme, information:

Konsole, Breeze light, information:

Kate, Breeze light, positive:

Kate, Breeze dark, information:

Reviewers: hein, Plasma, VDG, elvisangelaccio, cfeck

Reviewed By: hein, Plasma, cfeck

Subscribers: broulik, anemeth, abetts, cfeck, mart, fabianr, elvisangelaccio, jnoack, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12508