WIP: improvements to left sidebar

[astrofrog]

This is a work in progress and tries to improve the UI in the left sidebar (as per #1308). At the moment this:

• Makes it impossible for individual subsets to be out of sync with their group since GroupedSubsets no longer own their own style (but just refer to the parent subsetgroup)
• No longer make it possible to show subsets for specific datasets in the top left data collection but instead just show the subset group and allow the subset groups to be dragged and dropped onto viewers.

I still need to:

• Group subset layers in layer list
• Disable checkbox not whole layer in list

(and other things as I continue to improve this)