ui: Use crossfades for stack transitions

This will give Fractal-Next a nice bit of polish
by using clean transitions for every stack.

data/resources/ui/content.ui

@@ -13,6 +13,7 @@
         </child>
         <child>
           <object class="GtkStack" id="stack">
+            <property name="transition-type">crossfade</property>
             <child>
               <object class="ContentRoomHistory" id="room_history">
                 <property name="compact" bind-source="Content" bind-property="compact" bind-flags="sync-create"/>

data/resources/ui/login.ui

@@ -40,6 +40,7 @@
         </child>
         <child>
           <object class="GtkStack" id="main_stack">
+            <property name="transition-type">crossfade</property>
             <child>
               <object class="GtkStackPage">
                 <property name="name">greeter</property>

data/resources/ui/spinner-button.ui

@@ -3,6 +3,7 @@
   <template class="SpinnerButton" parent="GtkButton">
     <property name="child">
       <object class="GtkStack" id="stack">
+        <property name="transition-type">crossfade</property>
         <child>
           <object class="GtkLabel" id="label">
             <property name="use_underline">True</property>