fix tracks bugs for labeling and remove Grob class subclass, this may be confusing

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/ggbio@76771 bc3139a8-67e5-0310-9ffc-ced21a209358
Add combination methods for tracks and add get_gtable function to get gtable from tracks object. 

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/ggbio@76784 bc3139a8-67e5-0310-9ffc-ced21a209358
add cbind/rbind for tracks object and change top margin to 1

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/ggbio@76785 bc3139a8-67e5-0310-9ffc-ced21a209358