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

crates/workspace/src/workspace.rs

@@ -1387,7 +1387,9 @@ impl Workspace {
             };
 
             if let Some(task) = this
-                .update(&mut cx, |this, cx| this.open_workspace_for_paths(paths, cx))
+                .update(&mut cx, |this, cx| {
+                    this.open_workspace_for_paths(false, paths, cx)
+                })
                 .log_err()
             {
                 task.await.log_err();
@@ -1398,6 +1400,7 @@ impl Workspace {
 
     pub fn open_workspace_for_paths(
         &mut self,
+        replace_current_window: bool,
         paths: Vec<PathBuf>,
         cx: &mut ViewContext<Self>,
     ) -> Task<Result<()>> {
@@ -1405,7 +1408,10 @@ impl Workspace {
         let is_remote = self.project.read(cx).is_remote();
         let has_worktree = self.project.read(cx).worktrees().next().is_some();
         let has_dirty_items = self.items(cx).any(|item| item.is_dirty(cx));
-        let window_to_replace = if is_remote || has_worktree || has_dirty_items {
+
+        let window_to_replace = if replace_current_window {
+            window
+        } else if is_remote || has_worktree || has_dirty_items {
             None
         } else {
             window