Implement menu validation based on a string expression

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The system should implement a way to define a a menu validation based on an expression that contains a property, an operator and a expected value.

For instance, ${myproperty}=="true" would validate a <visibility> or <visibility> node only if the value of property myproperty is set to the value true.

The attribute sugested for specifying an expression would be propertyexpression.
For instance:

<visibility propertyexpression="&quot;%{myproperty}&quot; == &quot;&quot;" />

This kind of validation would allow the application to support "exclusive menus". For instance, in a 2 menu scenario, the application would be able to only show one of the two menus at a time.
For instance, the following could be implemented:

  <menu name="A">
    <visibility propertyexpression="&quot;%{next_menu}&quot; == &quot;&quot; || &quot;%{next_menu}&quot; == &quot;A&quot;" />
    <actions>
      <!--Do something meaningful-->
      
      <!--Change the property to only show B.-->
      <property name="next_menu" value="B" />
    </actions>
  </menu>
  <menu name="B">
    <visibility propertyexpression="&quot;%{next_menu}&quot; == &quot;B&quot;" />
    <actions>
      <!--Do something meaningful-->

      <!--Change the property to only show C.-->
      <property name="next_menu" value="C" />
    </actions>
  </menu>
  <menu name="C">
    <visibility propertyexpression="&quot;%{next_menu}&quot; == &quot;C&quot;" />
    <actions>
      <!--Do something meaningful-->

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

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An external library may be required for evaluating the string expressions.
Possible candidates are the following:

• C++ Mathematical Expression Library, MIT license.
• exprtk, MIT License
• StackOverflow discussion
• tinyexpr, branch logic, ZLib License.

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More candidate:

• MathExpr, MIT license.
• cparse, MIT license.
• cschreib's fork of tinyexpr, See tinyexpr issue 29, MIT license.
• exprtk on Github, MIT license.

or code my own parser as described here.

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Some use case scenarios:

<visibility expression="${selection.count} == 3" />
<visibility expression="&quot;${my_software.active}&quot; == &quot;true&quot;" />
<visibility expression="&quot;%{level}&quot; == &quot;unknown&quot; || &quot;%{level}&quot; == &quot;top&quot;" />
<visibility expression="length(${selection.path}) >= 10" />
<visibility expression="'${myapp.state}' == 'PAUSED'" />

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Add a dependency to INSTALL.md:

[exprtk d312ba9](https://github.com/ArashPartow/exprtk) (downloaded automatically)

Add the missing license file to /licenses directory.
example available at https://raw.githubusercontent.com/ArashPartow/exprtk/master/exprtk.hpp