Externalisation of UnifiedViews theme

[farky]

refactoring which allows to develop and use themes separately from then UnifiedViews

• for creating new Theme is no longer necessary to create class which extends AppEntry
• name of Theme has to be set in frontend/config.properties in frontend.theme property
• all static content for Theme with name XYZ (frontend.theme = XYZ) has to be placed in VAADIN/themes/XYZ folder
• theme can be created separately without UnifiedViews and then copied to the deployed UV. set correctly frontend.theme property and afterward context restart is necessary

[skrchnavy]

@farky please finish feature