ui: Homogeneize the panels

This makes the right panel take 50% of the window when it is wide
enough, and make the two other panels share the remaining 50%.

src/client/application/application-main-window.vala

@@ -1351,7 +1351,6 @@ public class Application.MainWindow :
         );
 
         this.conversation_viewer.hexpand = true;
-        this.conversation_size_group.add_widget(this.conversation_viewer);
         this.conversation_viewer_box.add(this.conversation_viewer);
 
         // Main toolbar

ui/application-main-window.ui

@@ -27,7 +27,7 @@
                 <child>
                   <object class="HdyLeaflet" id="inner_leaflet">
                     <property name="visible">True</property>
-                    <property name="hexpand_set">True</property>
+                    <property name="hexpand">True</property>
                     <property name="can_swipe_back">True</property>
                     <property name="transition_type">over</property>
                     <signal name="notify::folded" handler="on_inner_leaflet_changed" swapped="no"/>
@@ -147,6 +147,7 @@
                 <child>
                   <object class="GtkBox" id="conversation_viewer_box">
                     <property name="visible">True</property>
+                    <property name="hexpand">True</property>
                     <property name="orientation">vertical</property>
                     <child>
                       <object class="GtkRevealer" id="conversation_viewer_actions_revealer">
@@ -195,6 +196,12 @@
       <class name="geary-main-window"/>
     </style>
   </template>
+  <object class="GtkSizeGroup">
+    <widgets>
+      <widget name="inner_leaflet"/>
+      <widget name="conversation_viewer_box"/>
+    </widgets>
+  </object>
   <object class="GtkSizeGroup" id="folder_size_group">
     <property name="mode">horizontal</property>
     <widgets>
@@ -221,6 +228,9 @@
   </object>
   <object class="GtkSizeGroup" id="conversation_size_group">
     <property name="mode">horizontal</property>
+    <widgets>
+      <widget name="conversation_viewer_box"/>
+    </widgets>
   </object>
   <object class="HdySwipeGroup" id="conversations_swipe_group">
     <swipeables>

ui/components-main-toolbar.ui

@@ -9,9 +9,9 @@
     <child>
       <object class="HdyLeaflet" id="conversations_leaflet">
         <property name="visible">True</property>
+        <property name="hexpand">True</property>
         <property name="can_swipe_back">True</property>
         <property name="transition_type">over</property>
-        <property name="hexpand_set">True</property>
         <child>
           <object class="HdyHeaderBar" id="folder_header">
             <property name="visible">True</property>
@@ -212,6 +212,12 @@
       </packing>
     </child>
   </template>
+  <object class="GtkSizeGroup">
+    <widgets>
+      <widget name="conversations_leaflet"/>
+      <widget name="conversation_header"/>
+    </widgets>
+  </object>
   <object class="HdyHeaderGroup" id="conversations_header_group">
     <property name="decorate-all" bind-source="conversations_leaflet" bind-property="folded" bind-flags="sync-create"/>
     <headerbars>