Support custom or default properties defined at load time.

[end2endzone]

It would be interesting to be able to define default properties (defined by the user) when a file is loaded.

The proposed method is as follows:

<?xml version="1.0" encoding="utf-8"?>
<root>
  <default>
    <property name="myapp.installation.date" value="2019-01-24" />
  </default>
  <shell>
  </shell>
</root>

The feature could also be usefull for implementing mutually exclusive menus.
For instance, by defining a default value of property display_menu_a to true, one can implement the following toggling menus:

  <menu name="A">
    <visibility properties="display_menu_a" />
    <actions>
      <!--Do something meaningful-->

      <!--Change the property to only show B.-->
      <property name="display_menu_a" value="" />
      <property name="display_menu_b" value="true" />
    </actions>
  </menu>
  <menu name="B">
    <visibility properties="display_menu_b" />
    <actions>
      <!--Do something meaningful-->

      <!--Change the property to only show A.-->
      <property name="display_menu_a" value="true" />
      <property name="display_menu_b" value="" />
    </actions>
  </menu>

[end2endzone]

Implemented in 33a7a29