feat: Port about window to adw::Dialog (Adw.Dialog)

Other Changes:

Change window height to 294 to fit the HIG:
https://developer.gnome.org/hig/guidelines/adaptive.html#small-size-handling

Change adw features version in `Cargo.toml` from `v1_4` to `v1_5` to use
the new dialog widget(s)

src/about.rs

@@ -21,11 +21,11 @@ use gettextrs::gettext;
 
 use crate::config;
 
-pub async fn window() -> adw::AboutWindow {
+pub async fn dialog() -> adw::AboutDialog {
     // Builders are a pattern that allow you to create
     // an object and set all relevant properties very
     // easily in a way that's idiomatic to Rust.
-    adw::AboutWindow::builder()
+    adw::AboutDialog::builder()
         .application_name(gettext("Image Viewer"))
         .application_icon(config::APP_ID)
         .version(config::VERSION)