IZPACK-1207, IZPACK-1208 - panel enhancements

This pull request addresses a couple of issues partly depending on on each other at once:

* https://jira.codehaus.org/browse/IZPACK-1207
Make TargetPanel.warn ("The directory already exists! Are you sure you want to install here and possibly overwrite existing files?") message optional
* https://jira.codehaus.org/browse/IZPACK-1208
Enhance panel configuration options (syntax, add conditions)
* XSD fixes for recent implementations

**IZPACK-1207**
On TargetPanel, there is a message {{TargetPanel.warn}} ("The directory already exists! Are you sure you want to install here and possibly overwrite existing files?") shown uncondtionally.

This might be inconvenient if you want to provide an update installer, which counts with existing files in the target directory.

An option would be needed for being able to deactivate this warning and continue wit the installation.

Note:
There is already a optional {{ShowCreateDirectoryMessage}} boolean IzPack variable used for controlling a similar warning whether the target directory should be created if it doesn't exist. This approach might be enough for now.

**IZPACK-1208**
In future, it should be possible to define panel {{configuration}} entries using the element name as the option name directly, similar to Maven.

Additionally there should be used optional conditions which must evaluate true before the given value applies.

The old syntax {{<param name="..." value="..." />}} will be preserved for compatibility reasons to not break older environments, but I will recommend to use the newer one.

Example:
```xml
<panel classname="TargetPanel" id="panel.target">
  <configuration>
    <!-- Don't show warning if target directory already exists -->
    <param name="ShowExistingDirectoryWarning" value="false" />
  </configuration>
</panel>
```
should be written now:
```xml
<panel classname="TargetPanel" id="panel.target">
  <configuration>
    <!-- Don't show warning if target directory already exists -->
    <ShowExistingDirectoryWarning>false</ShowExistingDirectoryWarning>
  </configuration>
</panel>
```
which can be enhanced by a condition:
```xml
<panel classname="TargetPanel" id="panel.target">
  <configuration>
    <!-- Don't show warning if target directory already exists -->
    <ShowExistingDirectoryWarning condition="isUninstaller|isUpdate">false</ShowExistingDirectoryWarning>
  </configuration>
</panel>
```

**XSD fixes**
There are several installation descriptor schema fixes I found for recent implementations