CSS: update syntax and fix some errors

Summary:

• fix tag followed by a pseudo-class in a nested rule (@media, ...)
• fix constraint in a rule
• fix Kate auto-completion for properties and rules

New highlighting categories

• Keyword
• Operator
• Separator Symbol
• SpecialChar
• Variable
• Value Keyword
• Color
• Number
• Unit
• Selector Tag

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Frameworks, Kate, Framework: Syntax Highlighting

Differential Revision: https://phabricator.kde.org/D14137