Add most of a "CloseButton" widget

I'm trying out a new scope strategy. The CloseButton exists as a real
struct with the `Component` trait. There's a bundle spawning utility
function and several observer systems. The observer systems have been
added directly to the app, which I *believe* means they won't be
duplicated the way the Big Red Button's observers will.

src/resources.rs

@@ -17,6 +17,17 @@ pub struct UiTheme {
   // pressed
   pub brb_pressed_bg: Color,
   pub brb_pressed_border: Color,
+
+  // Colors for low-priority buttons
+  // normal
+  pub quiet_bg: Color,
+  pub quiet_border: Color,
+  // hover
+  pub quiet_hover_bg: Color,
+  pub quiet_hover_border: Color,
+  // pressed
+  pub quiet_pressed_bg: Color,
+  pub quiet_pressed_border: Color,
 }
 
 impl FromWorld for UiTheme {
@@ -28,6 +39,13 @@ impl FromWorld for UiTheme {
       brb_hover_border: RED.into(),
       brb_pressed_bg: GREEN.into(),
       brb_pressed_border: DARK_GREEN.into(),
+
+      quiet_bg: GRAY.into(),
+      quiet_border: DARK_GRAY.into(),
+      quiet_hover_bg: DARK_GRAY.into(),
+      quiet_hover_border: LIGHT_GRAY.into(),
+      quiet_pressed_bg: LIGHT_GRAY.into(),
+      quiet_pressed_border: GRAY.into(),
     }
   }
 }