Improve new comparison UI (#590)

The main goal here is to separate the blank vs regular comparison in the
UI. This commit does two small things to try and achieve this:
 * we set the Compare action as the "suggested" action using CSS, which
   means the button should get a suggest-action colour hint; and
 * we set the Blank comparison action as secondary, meaning that it will
   get packed to the left hand side of the dialog.

Author: kaiw

meld/resources/ui/new-diff-tab.ui

@@ -458,6 +458,7 @@
                 <property name="expand">False</property>
                 <property name="fill">False</property>
                 <property name="position">0</property>
+                <property name="secondary">True</property>
               </packing>
             </child>
             <child>
@@ -470,6 +471,9 @@
                 <property name="receives-default">True</property>
                 <property name="use-underline">True</property>
                 <signal name="clicked" handler="on_button_compare_clicked" swapped="no"/>
+                <style>
+                  <class name="suggested-action"/>
+                </style>
               </object>
               <packing>
                 <property name="expand">False</property>