Optimization of FrontBase Plugin

[darkv]

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.