Lots of small improvements and fixes

New features in alpha 9

• Github workflow: create tickets and pull requests in Github!
• A list of breaking and incompatible changes in FW 8
• Introduce VaadinServiceInitListener
• Separated Binding and BindingBuilder
  • Changes existing API by modifying returned type in Binder
• Added shorthand Listing::setItems(Stream)
• Added shorthand HasValue::clear()
  • corresponds to field.setValue(field.getEmptyValue());
• Read-only support for Single- & Multiselection models in Grid.
• Make ViewChangeListener a functional interface by adding default implementation of afterViewChange
• Introduced shared state for every core Framework component, except AbstractExtension

Existing features added from version 7

• Allow to set caption for the empty selection in a ComboBox (V7)
• Added Grid::setSelectionMode(SelectionMode) shorthand for Grid
  • Changes existing API by hiding Grid::setSelectionModel
• Added a generic selection listener to Grid
  • More specific selection listeners added to SingleSelectionModel and MultiSelectionModel
• All changes introduced by 7.7.4 are migrated into master for FW 8 and compatibility components when applicable

Fixes in alpha 9

• Changed return type of Binder::getBean to BEAN instead of Optional<BEAN>
  • Changes existing API by modifying returned type in Binder
• Fix focus and blur handling for CheckBoxGroup, NativeSelect and RadioButtonGroup
• Remove HideAppendableDataProvider::combineFilters
• Don't use FW 8 FieldEvent classes in FW 7 compatibility module
• Deprecated Button::setHtmlContentAllowed
• Removed unused browser specific prefixes for box-sizing
• Fix BeanBinder to accept "raw generics type" for field bindings, e.g. Listings