Improve multiple selection UX

[kuychaco]

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

[kuychaco]

Elaborated on this in separate issues, with gifs for illustration purposes

#260 - Don't update file diff view when selecting multiple items
#210 - Allow user to select multiple items by shift+clicking
#261 - Support cmd-clicking to toggle selection state?

[kuychaco]

This issue is redundant