Adjust "recent projects" modal behavior to allow opening projects in both current and new window (#8267)

![image](https://github.com/zed-industries/zed/assets/2690773/7a0927e8-f32a-4502-8a8a-c7f8e5f325bb)

Fixes https://github.com/zed-industries/zed/issues/7419 by changing the
way "recent projects" modal confirm actions work:
* `menu::Confirm` now reuses the current window when opening a recent
project
* `menu::SecondaryConfirm` now opens a recent project in the new window 
* neither confirm tries to open the current project anymore
* modal's placeholder is adjusted to emphasize this behavior

Release Notes:

- Added a way to open recent projects in the new window