fix(axevent_example): Return Break from signal handlers (#125)

Benefits include:
- The signal handler is automatically unregistered and subsequent
  signals of the same type will cause the program to exit immediately.
- The space of possible behaviors suggested by the code better match
  the space of actual behaviors; I can find no literature and come up
  with no experiment that causes the signal handler to be called twice
  even when the signal handler returns continue.

Note however that:
- If a task is already running, the signal handler will not run until
  it has returned.
- If other tasks were already scheduled before the signal was received,
  these may still run before the main loop returns; I am unsure about
  the details.
- Other sources are not automatically cleaned up; if the `SourceId` is
  kept then we can call `.remove()` on this to clean them up.

---

`apps/axstorage_example/src/main.rs`:
- Use `unix_signal_add_once` because it is more concise.

apps-aarch64.checksum

@@ -1,4 +1,4 @@
-5ffc7c3b88bd966b8f0310ac7299ff05df37cfb9  target-aarch64/acap/axstorage_example_0_0_0_aarch64.eap
+0018f1c3b46545bebbef794b68900c254a7785e7  target-aarch64/acap/axstorage_example_0_0_0_aarch64.eap
 10227865e8dadfeeda0e4a3c1a0d88ed18bc019a  target-aarch64/acap/bounding_box_example_0_0_0_aarch64.eap
 27016805520639a6a1029661f1225f1f6df180fd  target-aarch64/acap/consume_analytics_metadata_0_0_0_aarch64.eap
 51f9e2ce6cfc81264afa310c9c51ee05ff45666d  target-aarch64/acap/embedded_web_page_0_0_0_aarch64.eap

apps-aarch64.filesize

@@ -1,4 +1,4 @@
-3694	target-aarch64/acap/axstorage_example_0_0_0_aarch64.eap
+3698	target-aarch64/acap/axstorage_example_0_0_0_aarch64.eap
 2213	target-aarch64/acap/bounding_box_example_0_0_0_aarch64.eap
 2083	target-aarch64/acap/consume_analytics_metadata_0_0_0_aarch64.eap
 859	target-aarch64/acap/embedded_web_page_0_0_0_aarch64.eap

apps/axstorage_example/src/main.rs

@@ -271,19 +271,13 @@ fn main() -> ExitCode {
 
     glib::timeout_add_seconds(10, || write_data("file1"));
     glib::timeout_add_seconds(10, || write_data("file2"));
-    glib::unix_signal_add(SIGTERM, {
+    glib::unix_signal_add_once(SIGTERM, {
         let main_loop = main_loop.clone();
-        move || {
-            main_loop.quit();
-            ControlFlow::Continue
-        }
+        move || main_loop.quit()
     });
-    glib::unix_signal_add(SIGINT, {
+    glib::unix_signal_add_once(SIGINT, {
         let main_loop = main_loop.clone();
-        move || {
-            main_loop.quit();
-            ControlFlow::Continue
-        }
+        move || main_loop.quit()
     });
 
     main_loop.run();