Add the option to only load the current frame(s)

[TimSylvester]

Pretty straightforward. Worst part is that changing the current frame can now produce changes and so has to be thread-managed, but I left the old call signature in for cases where you know that's not relevant.

I haven't really done anything with "focus," so I'm not sure I handled that correctly, but there's a current frame per focus so I treated a frame set as current on any focus as "active."

[sjg-wdw]

I wrote focus support for a certain company that got bought by Apple and never shipped it so 'meh'. I'm sure we'll find a use for it again.