Miscellaneous fixes

[astrofrog]

This includes the following fixes:

• Fixes the metadata dialog for non-FITS data
• Fixes a bug when converting from world to pixel coordinates which caused issues when using two cubes with different projections in the same image viewer
• Fixes a bug that occurred when in some reprojection cases an empty list/array of coordinates was passed to the world <-> pixel functions
• Fixes a bug that caused the image viewer slices to not be reset correctly when changing reference data
• Improves the single pixel selection mode to add crosshairs