Odd selection behavior

[alexsb]

When I select items, the behavior is not consistent with what is expected, e.g., from file browsers operating systems.

First, I'm not sure we need the separation of check marks and frames. I suppose check marks can be used without modifier keys, but there never should be a state where something has a frame but not a checkmark (see #459).

To Reproduce
CMD/CTRL Select is not implemented at all
Shift Select behaves inconsistent:

• Select an item (e.g., row 1)
• Shift click on a subsequent row (e.g., row 5)
  -> correct behavior: row 1-5 is selected
• Next click on row 10
  -> incorrect behavior: row 5-10 is selected
  -> expected behavior: row 1-10 is selected

See video:

Screen.Recording.2021-07-22.at.4.26.00.PM.mov

Expected behavior
For shift click, multiple clicks should always refer to the initial click selection, like here:

Screen.Recording.2021-07-22.at.4.28.15.PM.mov

For cmd-click, items should be added to or removed from the selection.

[sgratzl]

current CTRL behavior:

regarding shift behavior:
we have 5 rows: 1,2,3,4,5
what should be selected in the following cases:

• selected: 1, shift-click on 3 -> 1,2,3
• selected: 1,2, shift-click on 5 -> 1,2,3,4,5
• selected: 4, shift-click on 2 -> ?
• selected: 1,3 shift-click on 5 -> ?
• selected: 1,5, shift-click on 3 -> ?
• selected: 4,5, shift-click on 1 -> ?