Clicking on items in the working set seems slow

[njx]

Selecting items in the working set seems noticeably sluggish, at least on the Mac. I went back and looked at the previous sprint build, and it seems like there's a greater pause between when the filename updates in the title bar and the editor content updates.

Looking at the timeline, it seems like there's a lot of whole-document style recalculation and re-layout happening in updating the working set selection, and it's happening multiple times during a single click.

I don't think this is stop-ship for 0.44, but we should dig into this in 0.45 since it's a noticeable performance regression.

[njx]

(BTW, this is running off the release branch with the leaky listener fix. I'm not sure if this is something that gets worse over time - when I restarted Brackets it didn't seem quite as severe.)

[JeffryBooher]

@njx I made some serious improvements to the way way panes are focused and activated and delay creating the drag ghost until you drag so, if you want to give it a whirl, the changes are in jeff/wsv-perf-improvements