OPENDJ-1934 UI: Create/Update indexes

Add support for pluggable backend indexes in control panel

* org.opends.guitools.controlpanel.datamodel.ControlPanelInfo.java
** Add a method to initialize the new config framework

* org.opends.guitools.controlpanel.datamodel.IndexTypeDescriptor.java
** For each old/new configuration index type, creates a local attribute to replace existing and avoid new switch statements
** Add two new methods to retrieve a set of new configuration index types for a provided set of IndexDescriptor

* org.opends.guitools.controlpanel.ui.AbstractIndexPanel.java
** Add methods to create/update indexes offline using the new configuration framework

* org.opends.guitools.controlpanel.ui.IndexPanel.java
** Add the pluggable backend indexes support in existing modifyIndex(...) method
** Rename modifyIndex() to modifyIndexOffline()
** Rename modifyIndex(InitialLdapContext ctx) to modifyIndexOnline(InitialContext ctx)
** Change the method process to avoid hardcoded LDIF code (getIndexLDIF()) usage.
** Remove outdated comment

* org.opends.guitools.controlpanel.ui.NewIndexPanel.java
** Add the pluggable backend indexes support in existing createIndex(...) method
** Rename createIndex() to createIndexOffline()
** Rename createIndex(InitialLdapContext ctx) to createIndexOnline(InitialLdapContext ctx)
** Change the method process to avoid hardcoded LDIF code (getIndexLDIF()) usage.
** Remove outdated comment