Reduce number of threads in ui layer

[herrgrossmann]

The ui layer is very lavish with creation of threads.
Even is these thread sleeping most of the time, it makes it difficult to monitor the application, e.g. with jvisualvm.

The goal is to share executorService instances among the classes they was needed.

Candidates can be found in the following classes:

• org.jowidgets.cap.ui.impl.ParentSelectionListener
• org.jowidgets.cap.ui.impl.widgets.BeanTableSearchFilterToolbar
• org.jowidgets.cap.ui.impl.widgets.BeanTableImpl
• org.jowidgets.cap.ui.impl.BeanTableModelImpl
  ...