Improve multiple selection UX #241

Don't update FilePatchView as items are added to collection

MultiListCollection state

• currently selected items
• current tail
• current head

Click:

• select item
• set tail to the item

Shift-click / shift-up/down / drag

• Set head to item
• Add all items between tail and head to selection

Cmd-click:

• toggle item in selection or not
• if item is head/tail, move head/tail to the next/prev item
• else set item as new tail

Cmd-drag:

• add to current selection

JOINING SELECTION
head/tail seems to be "sucked" to the other end of the list