Enable basic functionality for channel name editing and import + refine cell types controls

What

Cell type colors now mix properly (for cells with multiple cell types as well as overlapping cells)
Hovering over cells now show which cell type(s) that cell is labeled with
Implement remove cells mode
Added instructions for cell type labeling
Deleting cells deletes them from cell types now
Cell types are separately tracked by feature (segmentation)
Color map bug fixes
Channel names can now be edited
Display controls visually tweaked
Cell type opacity can be toggled and adjusted
Channel name can be imported from OME.tiff metadata