Restore read coverage support

These commits should allow the webcode to use read coverage data from the variation DB again.

The changes to TextSequence.pm alter the way the sequence is rendered slightly; lowercase sequence is now allowed and represents sequence in the secondary strain that is assumed to be the same as the reference though has no read coverage data to confirm that.

caf94c6 reverts me accidentally including some code in TextSequence.pm that had been moved to Component.pm by Anne (it doesn't show up in the summary below).

NB this working may depend on some code going into core which may or may not have been merged in by the time you merge this.