* Added initial versions of the Exercise dialog controller class and FXML layout definiton.
* Exercise dialog: completed Main tab in FXML layout definiton.
* Exercise dialog: added widget IDs to FXML layout definiton.
* Exercise dialog: completed Optional tab in FXML layout definiton.
* Exercise dialog: removed duplicated ID in FXML layout definiton.
* Exercise dialog: completed FXML layout definiton.
* Added dependency of backport library openjfx-dialog 1.0.2
  (will be part of official Java 8u40)
* Use OpenJFX dialog API for for all JavaFX based dialog
  => less migration effort when Java 8u40 will be released
* Renamed *Model classes in Dialog controllers to *ViewModel (MVVM)
* Added workaround for too big fonts in custom dialog content.
* TODO updates.
* TODO updates.