Avoid layers being reordered when dealing with groups

[Keavon]

• Grouping (with CtrlG):

  • Fix external consistency of grouping layers so they don't jump to the top of the hierarchy #1627:
    External consistency: Grouping the selected layers causes the group to be inserted at the top of the layer stack. It should go just above where the layer selected layers were located.
    The current incorrect behavior:

    	-> CtrlG ->

  • Fix internal consistency of layer ordering when grouping #1645
    Internal consistency: Grouping layers should also preserve the order that they were sorted once those layers end up in the new group
    The current incorrect behavior:

    	-> CtrlG ->

• Ungrouping (with CtrlShiftG):

  • Fix ungrouping bugs to preserve layer ordering and prevent incorrect layer deletion #1672
    External consistency: Ungrouping should also preserve order by placing the content in the hierarchy where the group used to be.
    The current incorrect behavior:

    	-> CtrlShiftG ->

  • Fix internal consistency of layer ordering when grouping #1645
    Internal consistency: While ungrouping, the content in the group itself should also be have its order preserved amongst its grouped siblings.
    The current incorrect behavior:

    	-> CtrlShiftG ->

• Creating a new folder (with CtrlShiftN while in fullscreen mode of the web app, or the folder button in the top bar of the Layers panel):

  • Place new folders in the correct stack order relative to other selected layers #1678
    Should be placed directly above the topmost selected layer in the hierarchy (at the level of the least nested layer)
    The current incorrect behavior:

    	-> this button is clicked ->