Support replacing angle brackets in SelectView popup button

Allow a SelectView to be configured with custom decorators instead of the
angle brackets around the current item label.

cursive-core/src/views/select_view.rs

@@ -72,6 +72,8 @@ pub struct SelectView<T = String> {
 
     // `true` if we show a one-line view, with popup on selection.
     popup: bool,
+    // Decorators to draw around the popup button.
+    decorators: [String; 2],
 
     // We need the last offset to place the popup window
     // We "cache" it during the draw, so we need interior mutability.
@@ -102,6 +104,7 @@ impl<T: 'static + Send + Sync> SelectView<T> {
             on_submit: None,
             align: Align::top_left(),
             popup: false,
+            decorators: ["<".to_string(), ">".to_string()],
             autojump: false,
             last_offset: Mutex::new(Vec2::zero()),
             last_size: Vec2::zero(),
@@ -167,6 +170,19 @@ impl<T: 'static + Send + Sync> SelectView<T> {
         self.last_required_size = None;
     }
 
+    /// Use custom decorators around the popup button instead of "<" and ">".
+    ///
+    /// Chainable variant.
+    #[must_use]
+    pub fn decorators<S: Into<String>>(self, start: S, end: S) -> Self {
+        self.with(|s| s.set_decorators(start, end))
+    }
+
+    /// Use custom decorators around the popup button instead of "<" and ">".
+    pub fn set_decorators<S: Into<String>>(&mut self, start: S, end: S) {
+        self.decorators = [start.into(), end.into()];
+    }
+
     /// Sets a callback to be used when an item is selected.
     #[crate::callback_helpers]
     pub fn set_on_select<F>(&mut self, cb: F)
@@ -965,13 +981,16 @@ impl<T: 'static + Send + Sync> View for SelectView<T> {
             printer.with_style(style, |printer| {
                 // Prepare the entire background
                 printer.print_hline((1, 0), x, " ");
-                // Draw the borders
-                printer.print((0, 0), "<");
-                printer.print((x, 0), ">");
+                // Draw the decorators
+                printer.print((0, 0), &self.decorators[0]);
+                printer.print((x + 1 - self.decorators[1].len(), 0), &self.decorators[1]);
 
                 if let Some(label) = self.items.get(focus).map(|item| &item.label) {
                     // And center the text?
-                    let offset = HAlign::Center.get_offset(label.width(), x + 1);
+                    let offset = HAlign::Center.get_offset(
+                        label.width(),
+                        x + 1 - self.decorators.iter().map(|d| d.len()).sum::<usize>(),
+                    ) + self.decorators[0].len();
 
                     printer.print_styled((offset, 0), label);
                 }
@@ -1027,7 +1046,10 @@ impl<T: 'static + Send + Sync> View for SelectView<T> {
             .max()
             .unwrap_or(1);
         let size = if self.popup {
-            Vec2::new(w + 2, 1)
+            Vec2::new(
+                w + self.decorators.iter().map(|d| d.len()).sum::<usize>(),
+                1,
+            )
         } else {
             let h = self.items.len();