Improve layout in left panel

[astrofrog]

We need to improve the layout of the data collection and plot layers in the left panel. Following in-person discussions, we came up with the following plan:

• The datasets in the data collection should have an icon that relates to the dimensionality of the data. 1D datasets should have a table-like icon, 2D datasets should have an 'image' icon, and 3D+ datasets should have a cube. These should be color-coded according to the dataset color.

• We should get rid of the drop-down for each subset in the data collection. Instead, dragging a top-level subset group onto a viewer will just add the subsets for all datasets that are present in the plot. And adding a dataset (e.g. a 1D dataset onto an image) should always add all subsets too.

• In the plot layers, subsets should be grouped into a single entry with a drop-down option, and a top-level checkbox to simply hide all layers for a given subset (but when clicking on the drop-down, the visibility of each layer could be controlled too).

• Subset layers that can't be computed should be grayed out or hidden altogether

• Get rid of custom icon for image layers (formerly a galaxy). Icons should always be e.g. a colored square except if the layer uses markers in which case the marker should be shown.

• When right-clicking on a data, should have a way to edit the style (not just double clicking)

• When changing the style of a dataset, the marker reverts to a square

• We should explore options for showing/collapsing sections on the left, or using tabs for example (or multiple columns).