[IZPACK-1066] - Improve usability of resolving system properties as IzPack variables

Currently it is possible to resolve variables with the syntax ${SYSTEM_variablename} from system variables, in that case would be variablename resolved from the Java system properties as IzPack variable SYSTEM_variablename.
The implementation is very limited. For example the following code doesn't currently work at all:

```xml
<variables>
  <variable name="featureEnabled" value="${SYSTEM_featureEnabled}" />
</variables>

<conditions>
  <condition type="variable" id="isFeatureEnabled">
    <name>featureEnabled</name>
    <value>true</value>
  </condition>
</conditions>
```

but must be written as:

```xml
<conditions>
  <condition type="variable" id="isFeatureEnabled">
    <name>SYSTEM_featureEnabled</name>
    <value>true</value>
  </condition>
</conditions>
```

Furthermore, variables starting on the prefix SYSTEM_ are not really dynamically resolved, but all system variables are expanded at IzPack variables by default. This takes just time and resources during installing. System properties should be resolved on demand like all other variable types.

Added syntax ${SYSTEM[variable.Name]} for parsing system properties.
Marking old syntax ${SYSTEM_variable_Name} as obsolete, to be removed in one of the next major versions. Leavin it now for backward compatibility.