Move FilePicker::render from Component impl to normal impl

Merges the code for the Picker and FilePicker into a single Picker that
can show a file preview if a preview callback is provided. This change
was mainly made to facilitate refactoring out a simple skeleton of a
picker that does not do any filtering to be reused in a normal Picker
and a DynamicPicker (see helix-editor#5714; in particular [mikes-comment] and
[gokuls-comment]).

The crux of the issue is that a picker maintains a list of predefined
options (eg. list of files in the directory) and (re-)filters them every
time the picker prompt changes, while a dynamic picker (eg. interactive
global search, helix-editor#4687) recalculates the full list of options on every
prompt change. Using a filtering picker to drive a dynamic picker hence
does duplicate work of filtering thousands of matches for no reason. It
could also cause problems like interfering with the regex pattern in the
global search.

I tried to directly extract a PickerBase to be reused in Picker and
FilePicker and DynamicPicker, but the problem is that DynamicPicker is
actually a DynamicFilePicker (i.e. it can preview file contents) which
means we would need PickerBase, Picker, FilePicker, DynamicPicker and
DynamicFilePicker and then another way of sharing the previewing code
between a FilePicker and a DynamicFilePicker. By merging Picker and
FilePicker into Picker, we only need PickerBase, Picker and
DynamicPicker.

[gokuls-comment]: helix-editor#5714 (comment)
[mikes-comment]: helix-editor#5714 (comment)

helix-term/src/ui/picker.rs

@@ -120,7 +120,7 @@ impl Preview<'_, '_> {
     }
 }
 
-impl<T: Item> FilePicker<T> {
+impl<T: Item + 'static> FilePicker<T> {
     pub fn new(
         options: Vec<T>,
         editor_data: T::Data,
@@ -227,10 +227,8 @@ impl<T: Item> FilePicker<T> {
 
         EventResult::Consumed(None)
     }
-}
 
-impl<T: Item + 'static> Component for FilePicker<T> {
-    fn render(&mut self, area: Rect, surface: &mut Surface, cx: &mut Context) {
+    fn render_picker(&mut self, area: Rect, surface: &mut Surface, cx: &mut Context) {
         // +---------+ +---------+
         // |prompt   | |preview  |
         // +---------+ |         |
@@ -345,6 +343,12 @@ impl<T: Item + 'static> Component for FilePicker<T> {
             );
         }
     }
+}
+
+impl<T: Item + 'static> Component for FilePicker<T> {
+    fn render(&mut self, area: Rect, surface: &mut Surface, cx: &mut Context) {
+        self.render_picker(area, surface, cx)
+    }
 
     fn handle_event(&mut self, event: &Event, ctx: &mut Context) -> EventResult {
         if let Event::IdleTimeout = event {