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/theme/src/theme.rs

@@ -204,11 +204,12 @@ pub struct ChatPanel {
     pub hovered_sign_in_prompt: TextStyle,
 }
 
-#[derive(Debug, Deserialize, Default)]
+#[derive(Deserialize, Default)]
 pub struct ProjectPanel {
     #[serde(flatten)]
     pub container: ContainerStyle,
     pub entry: Interactive<ProjectPanelEntry>,
+    pub filename_editor: FieldEditor,
     pub indent_width: f32,
 }