Optimization of FrontBase Plugin

This patch makes the following changes to the FrontBase Plugin:

* SimpleDateFormatters were created every time an attribute had to be formatted, now those formatters are created only once per thread (using ThreadLocal as SimpleDateFormat is not thread safe)
* the class FrontBaseTypes exists in Wonder but apparently was nowhere used. As it makes sense to separate that data from the quite big plugin class those are used in favor
* add some generics

The value type constants as well as the method ```internalTypeForExternal(…)``` within the plugin class are only deprecated as there could be users subclassing the plugin and using them. With a later Wonder release those should be removed eventually.