Handle first click on Zed window

[danielzsh]

Fixes #4336

Release Notes:

• Zed now responds to the first, focusing, click on the application window.

[danielzsh]

@mikayla-maki I refactored the first_mouse handling into MouseDownEvent, what do you think?

[mikayla-maki]

This came out pretty nice! Let me test drive this a bit tomorrow and see how it feels.

[mikayla-maki]

Gave this a test drive, it seems like this has caused a pretty nasty bug where first clicks are treated like a drag event. In this example video, I only click on the terminal-editor border, and I only click on the assets folder, but both cause dragging behavior.

BugExample.mov

[danielzsh]

Oh yikes, I only tested double clicking the files; thanks for the catch!

[mikayla-maki]

This works great, thank you!

[mikayla-maki]

A good follow up for this PR might be making non-focused hovers work. I noticed that firefox does do a little styling of non-focused tabs you hover over, and I think that would be good behavior for things like the status bar buttons.