Create a function which safely initializes GTK once

This is only available in test code, and it allows GUI component tests to run without having to worry about double-initializing GTK

fitnesstrax/app/src/gtk_init.rs

@@ -0,0 +1,10 @@
+use std::sync::Once;
+
+static INITIALIZED: Once = Once::new();
+
+pub fn gtk_init() {
+    INITIALIZED.call_once(|| {
+        eprintln!("initializing GTK");
+        let _ = gtk::init();
+    });
+}

fitnesstrax/app/src/main.rs

@@ -17,6 +17,8 @@ You should have received a copy of the GNU General Public License along with Fit
 mod app;
 mod app_window;
 mod components;
+#[cfg(test)]
+mod gtk_init;
 mod types;
 mod view_models;
 mod views;