ui: clarify systray label

Systray is an old technology generally not shown by default.
However the behavior still exists. Now, if the app is closed, Jami
is running in background unless the option is disabled

Change-Id: I692bd3b78047da758ac64a17272eeba965e0ae27

ui/generalsettingsview.ui

@@ -129,7 +129,7 @@
                               <object class="GtkLabel" id="label_systray">
                                 <property name="visible">True</property>
                                 <property name="can_focus">False</property>
-                                <property name="label" translatable="yes">Show Jami icon (systray) in the systray area</property>
+                                <property name="label" translatable="yes">Keep Jami active when window is closed.</property>
                               </object>
                             </child>
                             <child>