Revise the main window design

src/resources/ui/gg-window.ui

@@ -6,6 +6,7 @@
     <property name="can_focus">False</property>
     <child type="titlebar">
       <object class="GtkHeaderBar" id="header_bar">
+        <property name="show-close-button">True</property>
       </object>
     </child>
     <child>
@@ -15,10 +16,11 @@
             <child>
               <object class="GtkBox">
                 <property name="orientation">vertical</property>
+                <property name="expand">True</property>
                 <child>
                   <object class="GtkImage">
-                    <property name="yalign">1</property>
-                    <property name="ypad">1</property>
+                    <property name="halign">center</property>
+                    <property name="valign">end</property>
                     <property name="icon-name">system-software-install-symbolic</property>
                     <property name="icon-size">6</property>
                   </object>
@@ -31,7 +33,7 @@
                   <object class="GtkLabel" id="no_instance_label">
                     <!-- This label gets overwritten in window.vala with an easily translatable markup -->
                     <property name="label" translatable="no">No instances configured. Go to preferences to set things up!</property>
-                    <property name="yalign">0.3</property>
+                    <property name="valign">start</property>
                   </object>
                   <packing>
                     <property name="expand">True</property>
@@ -44,7 +46,7 @@
                     <property name="action_name">app.preferences</property>
                   </object>
                   <packing>
-                    <property name="expand">False</property>
+                    <property name="expand">True</property>
                     <property name="fill">False</property>
                   </packing>
                 </child>