Use KF standard scheme for the include headers install location

- CamelCase fowarding headers in KF*/KSyntaxHighlighting/KSyntaxHighlighting
- Lowercase headers and _export.h in KF*/KSyntaxHighlighting/ksyntaxhighlighting
- _version.h KF*/KSyntaxHighlighting
- Remove <prefix>/include/KF*/ from the interface include dirs, now all
  headers are in KF*/KSyntaxHighlighting/

For backwards-compatibility, install all the headers in

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merge request