Add scale to BAM file coverage track

[Xewar313]

Add scale to BAM track, similar to the one used by BigWig track

[iromeo]

Instead of the word Coverage more useful is to make focus that for PE library we show 'Fragments' coverage and for SE library Reads coverage. I think word Coverage could be omitted. And also if track height is not big enough to show title it is ok not to show it at all