Improve the appearance of project panel filename editor

* Always layout single-line editors with a fixed height
* Preserve directory chevron when editing folder names
* Allow theming the filename editor

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/project/src/worktree.rs

@@ -1568,7 +1568,7 @@ pub struct Entry {
     pub is_ignored: bool,
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum EntryKind {
     PendingDir,
     Dir,