chore: move some components to custom ui crate (#10)

Reviewed-on: https://git.reya.su/reya/signed/pulls/10

crates/signed_ui/src/placeholder.rs

@@ -0,0 +1,19 @@
+use gpui::prelude::*;
+use gpui::{AnyElement, App, div};
+use gpui_component::{ActiveTheme, v_flex};
+
+/// A centered muted placeholder message, filling its parent.
+pub fn placeholder(message: &str, cx: &App) -> AnyElement {
+    v_flex()
+        .size_full()
+        .items_center()
+        .justify_center()
+        .p_4()
+        .child(
+            div()
+                .text_sm()
+                .text_color(cx.theme().muted_foreground)
+                .child(message.to_string()),
+        )
+        .into_any_element()
+}