Add possibility to create new loops

[Daniel Kratzert]

Probably a list of cif keys (not currently used in loops) with a, "append" button.

Append will create a loop and more rows can be added as usual.

There could be also a "add column" button?

[Daniel Kratzert]

• Show listwidget with list of all cif keys
• Make listwidget searchable (typing reduces list items)
• Button: "Add Column"
  • Adds current key to a second list
  • Second list is orderabe
  • Has button "Remove Column"
• Button: "Save Loop"
  • Adds loop to cif and shows new tab
  • use Appwindow().new_loop_tab(self, loop: gemmi.cif.Loop, num: int, tags: List[str])