general: Add placeholder text to search entries

Acc. HIG, "text fields should have placeholder text or a label".

https://developer.gnome.org/hig/patterns/controls/text-fields.html

src/gui/gcal-search-button.ui

@@ -50,6 +50,7 @@
             <property name="name">entry</property>
             <property name="child">
               <object class="GtkSearchEntry" id="entry">
+                <property name="placeholder-text" translatable="yes">Search events</property>
                 <property name="max-width-chars">0</property>
                 <property name="width-chars">0</property>
                 <signal name="activate" handler="on_entry_activate_cb" object="GcalSearchButton" swapped="no" />

src/gui/gcal-weather-settings.ui

@@ -43,6 +43,7 @@
     </child>
     <child>
       <object class="GtkEntry" id="weather_location_entry">
+        <property name="placeholder-text" translatable="yes">Search locations</property>
         <property name="primary_icon_name">edit-find-symbolic</property>
         <property name="primary_icon_activatable">False</property>
         <property name="primary_icon_sensitive">False</property>