Gt seqabstract

This cleans up the `GtSeqabstract` class and changes how it is used by xdrop which is its main usage right now.

A GtSeqabstract is now a fixed window onto another sequence which can be either `GtEncseq` or simple `GtUchar[]` accessing the underlying Sequnce outside of the window bounds results in an error as resetting the windows position is relatively cheap.

depends on pull request #228