ui: click TextBox to focus, fixes #415 (#475)

Previously text boxes could only be focused by cycling through
focus with the tab key, now you can click with the mouse as well.

src/ui/mod.rs

@@ -379,10 +379,19 @@ impl Container {
         let mx = (x / width) * SCALED_WIDTH;
         let my = (y / height) * SCALED_HEIGHT;
 
+        let mut clicked_element: Option<&Element> = None;
         for e in &self.elements {
             let r = Self::compute_draw_region(e, sw, sh, &SCREEN);
             if mx >= r.x && mx <= r.x + r.w && my >= r.y && my <= r.y + r.h {
                 e.click_at(&r, game, mx, my, sw, sh);
+                clicked_element = Some(e);
+            }
+        }
+
+        if let Some(e) = clicked_element {
+            if let Element::TextBox(_) = e {
+                self.clear_focus();
+                e.set_focused(true);
             }
         }
     }
@@ -391,6 +400,12 @@ impl Container {
         self.focusable_elements.push(el);
     }
 
+    fn clear_focus(&self) {
+        for e in &self.elements {
+            e.set_focused(false);
+        }
+    }
+
     pub fn cycle_focus(&mut self) {
         if self.focusable_elements.is_empty() {
             return;