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/window.ui

@@ -60,7 +60,7 @@
     <property name="default-width">600</property>
     <property name="default-height">498</property>
     <property name="width-request">360</property>
-    <property name="height-request">360</property>
+    <property name="height-request">294</property>
     <child>
       <object class="AdwBreakpoint">
         <condition>max-width: 590sp</condition>
@@ -220,4 +220,4 @@
       </object>
     </property>
   </template>
-</interface>
+</interface>