[KRDB] Fix wrong names in GTK colordef exports

Summary: The background & foreground color states of the button color variants were exporting to the wrong name. This revision changes the names to the ones used by Breeze GTK.

Test Plan:
Ensure that the button variant colors export properly now (backdrop [unfocused window], insensitive [disabled])

Before:
{F7176690}

After:
{F7176687}

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22992