Refactored CssImportPreProcessor and CssImportInspector

To be able to override the import-algorithm in LessCssImportPreProcessor,
the searching for @import-statements has to be moved into the method
doTransform. Also, some private attributes of AbstractCssImportPreProcessor
has to be accessible in derived classes and the Matcher, that is instanciated
in CssImportInspector, has to be accessable from other classes.
LessCssImportPreProcessor handles nested import-statements and media-queries

In LESS, it is allowed, that import-statements are nested in media-queries.
To avoid corrumption of the LESS-code, the imported code has to be placed
exactly, where the import-statements was noted.

LESS automatically generates according media-queries, if an import-statment
has a media-query. To mimic this behaviour, the LessCssImportPreProcessor
now nests the imported code inside an according media-query, if an import-
statement has a media-query.

TODO:
CssImportPreProcessor should be aware of media-queries as well. Otherwise,
it will corrupt CSS-code with import-statements, that are using media-
queries, as shown in the result of the added test-case
testImportWithMediaQuery.css