Gettext: Add "Translated String" style and spellChecking attribute

Summary:
New style ("Translated String") for a string associated with msgstr. Normal String and some other styles are now spellChecking=false.

Hardcoded colors are removed.

BUG: 392612

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23513