Gettext: fix "previous untranslated string" and other improvements/fixes

Summary:

• fix #| (previous untranslated string)
• fix hexadecimal entity
• fix html tag in a < b
• use <keyword> rather than <RegExpr/StringDetect> for auto-completion
• string without " at the end of the line is an error
• # followed by something other than " |:.," is an extension

Reviewers: Framework: Syntax Highlighting, dhaumann, cullmann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D23135