Say when a newer release is out, and stop there

A check on the releases page once a day: at start, on a timer while Dikte
runs, from the General tab on demand, and from `dikte update` at a terminal.
What it finds goes in the tray menu and in one notification per version, and
opens the release page.

Nothing is downloaded and nothing is installed. The four downloads are
installed four different ways and three of those belong to the platform: a
Mac bundle cannot rewrite itself while it is running, the Windows setup has
an uninstall entry of its own, an AppImage is a file kept wherever its owner
keeps it, and a checkout is updated with git. Being wrong about any one of
them means an installation somebody has to repair by hand.

Versions are compared by their numbers alone. A build off master carries the
released number with its commit after it, and that build is ahead of the
release it names rather than behind it; read as a version suffix, every
nightly would be told to go back to a release it had already passed.

The clock lives in its own file rather than in the settings, since a check
runs while the settings window may be open and a background write into
config.json is what would overwrite whatever it holds.

tests/test_ui.py

@@ -25,6 +25,7 @@ from dikte import ipc
 from dikte import overlay as overlay_module
 from dikte import paste
 from dikte import settings_ui
+from dikte import update
 from tests.support import DikteTest, only_these_tools
 
 # One application for the whole run; Qt allows no second one.
@@ -103,6 +104,7 @@ CHANGED = {
     "pause_shortcut": "Meta+P",
     "evdev_hotkey": True,
     "history_limit": 50,
+    "update_check": False,
 }
 
 
@@ -252,6 +254,31 @@ class Settings(DikteTest):
                 self.assertEqual(shown, [provider])
                 self.assertFalse(box.isHidden())
 
+    def test_the_update_line_names_the_version_that_is_running(self):
+        window = self.window(cfg.Config())
+        self.assertIn(settings_ui.__version__, window.update_status.text())
+        # Nothing to open until a check has found something to open.
+        self.assertTrue(window.update_page.isHidden())
+
+    def test_a_newer_release_puts_the_page_button_on_screen(self):
+        window = self.window(cfg.Config())
+        told = []
+        window.update_found.connect(told.append)
+        release = update.Release("9.9.9", "https://example.invalid/9.9.9", "")
+        window._on_update_checked(release, "")
+        self.assertIn("9.9.9", window.update_status.text())
+        self.assertFalse(window.update_page.isHidden())
+        self.assertEqual(window._release_url, release.url)
+        # And the tray hears about it from here rather than waiting a day.
+        self.assertEqual(told, [release])
+
+    def test_a_check_that_failed_says_why_and_hands_the_button_back(self):
+        window = self.window(cfg.Config())
+        window.update_now.setEnabled(False)
+        window._on_update_checked(None, "api.github.com answered HTTP 403.")
+        self.assertIn("403", window.update_status.text())
+        self.assertTrue(window.update_now.isEnabled())
+
     def test_the_settings_the_window_does_not_show_are_left_alone(self):
         """A tab nobody wrote must not reset what the command line set."""
         self.write_config({"silence_db": -42.0, "speech_margin_db": 15.0,