Add Unassign All to Resource Menu

[hot oriole]

We need the following buttons in resource screen:

• unassign all
• unassign all (this city only)
• focus this city: one click on a resource removes it, one click on a resource in the pool assigns it, clicking resources in other cities moves them to the focused city unless full. canceled by clicking again or clicking other buttons.

[short gorge]

shift click and control click for rapidly assigning multiple resources too