docs(examples): add tkinter file browser example

This adds an example that uses tkinter for the selection of a directory.

Signed-off-by: Patrick Avery <patrick.avery@kitware.com>

examples/validation/core/21_file_browser.py

@@ -0,0 +1,65 @@
+from tkinter import filedialog, Tk
+
+from trame.app import get_server
+from trame.ui.vuetify import SinglePageLayout
+from trame.widgets import trame, vuetify
+
+# -----------------------------------------------------------------------------
+# Trame setup
+# -----------------------------------------------------------------------------
+
+server = get_server()
+state, ctrl = server.state, server.controller
+
+# Keep track of the currently selected directory
+state.selected_dir = "None"
+
+# Ensure the tkinter main window is hidden
+Tk().withdraw()
+
+
+def open_directory():
+    kwargs = {
+        "title": "Select Directory",
+    }
+    dirpath = filedialog.askdirectory(**kwargs)
+    if not dirpath:
+        # User canceled.
+        print("Canceled")
+        return
+
+    print("Selected directory:", dirpath)
+    state.selected_dir = dirpath
+
+
+# The controller would let us set up callback logic in a separate file
+ctrl.open_directory = open_directory
+
+# -----------------------------------------------------------------------------
+# UI setup
+# -----------------------------------------------------------------------------
+
+layout = SinglePageLayout(server)
+
+with layout:
+
+    # Toolbar
+    with layout.toolbar as toolbar:
+        toolbar.clear()
+
+        vuetify.VSpacer()
+
+        vuetify.VBtn("Select Directory", click=ctrl.open_directory)
+
+        vuetify.VSpacer()
+
+        with layout.content:
+            with vuetify.VContainer() as container:
+                container.add_child("Selected Directory: {{ selected_dir }}")
+
+# -----------------------------------------------------------------------------
+# start server
+# -----------------------------------------------------------------------------
+
+if __name__ == "__main__":
+    server.start()