Data Table: single tab, bug fixes and subset input

[janezd]

Changing Data Table to single input will require refactoring settings, so this change can do several things at once (not doing so would require changing code that would be later thrown away):

• Change the input signal to a single table
• Preserve sorting (Sorting data in a way that is preserved when saving the workflow #6311)
• Preserve selection (Row number as a variable #6312)
• Add Data Subset input and indicate the subset in the table (Data Table needs a Data Subset input #6128)

[janezd]

Now what will happen to the selection and the sorting when the input data changes? I hope that if the selection was
set to "all", still all the data remains selected and the sorting order remains.

@wvdvegte, if nothing is selected, it has the same effect as if everything is selected.

... except that in this case sorting is not reflected in the output. Fixing this efficiently will require solving #5720, which looks simple but hides some nasty details.

[janezd]

Note: @ales-erjavec proposed to keep the existing widget but have it in a hidden category, so old workflows will keep working. For new workflows, only the new widget would be available.