window: hide windows on delete

As calls is supposed to run always in background,
destroying window on close results in unintended issues.
Let's simply hide the window instead.

src/ui/call-window.ui

@@ -9,6 +9,7 @@
     <property name="hide_titlebar_when_maximized">True</property>
     <property name="show_menubar">False</property>
     <property name="title" translatable="yes">Calls</property>
+    <signal name="delete-event" handler="gtk_widget_hide_on_delete"/>
     <child>
       <object class="GtkBox">
         <property name="visible">True</property>

src/ui/main-window.ui

@@ -8,6 +8,7 @@
     <property name="title" translatable="yes">Calls</property>
     <property name="hide_titlebar_when_maximized">True</property>
     <property name="show_menubar">False</property>
+    <signal name="delete-event" handler="gtk_widget_hide_on_delete"/>
     <child>
       <object class="GtkBox">
         <property name="visible">True</property>