ASE-GUI Edit Atoms and bugs

Atom table (merge request !3690 (merged) ):

• Edit atoms should have the shortcut "A".
• Changing atom indices
• Possible to toggle absolute and scaled positions in the atom table
• Column in atom table with constraints. Suggestion: "F" for fixed, "P" for fixed plane, "L" for fixed line, and a fourth option for the rest (maybe "O" for other)
• Maybe a column for masses.
• "Modify atoms" might be superceded by the table. Maybe the entire feature should take the name and shortcut of Modify atoms.

Bugs:

• Selected hidden atoms should still have outline
• When the main window is selected, it's possible to open several instances of other windows, e.g., Add atoms. There should only be one instance.