Add support for calling focus() on TextInput elements

This allows activating text inputs in signal handlers connected for
example to buttons.

This implements parts of #55

docs/builtin_elements.md

@@ -244,6 +244,9 @@ The `TextInput` is a lower-level item that shows text and allows entering text.
   within the item
 * **`has_focus`** (*bool*): Set to true when item is focused and receives keyboard events.
 
+### Methods
+
+* **`focus()`** Call this function to focus the text input and make it receive future keyboard events.
 
 ### Example
 

docs/langref.md

@@ -574,4 +574,22 @@ App := Window {
         label2 := LabeledInput {}
     }
 }
+```
+
+It's also possible to manually activate the focus on elements such as ```TextInput```:
+
+```60
+import { Button } from "sixtyfps_widgets.60";
+
+App := Window {
+    GridLayout {
+        Button {
+            text: "press me";
+            clicked => { input.focus(); }
+        }
+        input := TextInput {
+            text: "I am a text input field";
+        }
+    }
+}
 ```