[IZPACK-1115] Use <executable> and <parsable> with filesets which do not access the filesystem

The `<parsable>` tag currently supports a nested fileset, which requires a `dir` and a `targetDir` attribute. Accessing the filesystem a second time here is not a goot choice and this lack has been already commented in the code. Instead, `<parsable>` should mark files parsable, which are added using the `<file>`, `<singlefile>` and `<fileset>` tags nested against the `<pack>` definition, the original filesystem access should happen only there (the files are packed into the installer).

The same for the `<executable>` tag, which currently even doesn't support nested filesets although it is documented.

Furthermore, for all filesets (nested in `<pack>`, `<executable>`, `<parsable>`) the `targetDir` attribute should be optional and default to "$\{INSTALL_PATH}".

The dir attribute should no longer been parsed in `<fileset>` nested to `<executable>`, `<parsable>` at all.

Example:
```xml
<pack name="Core files" required="yes" id="pack.core" condition="Install">
  <description>Core files</description>
  <fileset dir="@{staging.dir}" override="true">
    <exclude name="*.zip" />
    <exclude name="conf/*.properties" />
    <exclude name="conf/*.xml" />
  </fileset>
  <fileset dir="@{staging.dir}/config_files" targetdir="${INSTALL_PATH}/conf" override="true" overrideRenameTo="*.configbak">
    <include name="*.properties" />
    <include name="*.xml" />
    <exclude name="special.xml" />
  </fileset>
  <parsable encoding="UTF-8">
    <fileset targetdir="${INSTALL_PATH}/conf">
      <include name="wrapper.conf" />
    </fileset>
  </parsable>
  <parsable>
    <fileset>
      <include name="**/*.bat" />
      <include name="**/*.cmd" />
    </fileset>
  </parsable>
  <parsable type="shell">
    <fileset>
      <include name="**/*.sh" />
    </fileset>
  </parsable>
  <executable>
    <fileset>
      <include name="**/*.sh" />
    </fileset>
  </executable>
</pack>
```