Taxonomy: Fix styling of dialog buttons

The dialogs in this plugin extend from AbstractDialog. This dialog uses the native browser buttons. In the current Chrome these buttons appear now very small.

Solution: extend from Dialog instead.

Dialog also provides some sugar for common use cases like setting the dialog size and title:

• instead of overridding AbstractDialog#getProperties to return a ValueMap with width&height key-value pairs, call Dialog#setSize(DialogContants.MEDIUM_AUTO) in the constructor (see DialogConstants for all available OOTB sizes)
• instead of overriding AbstractDialog#getTitle, call Dialog#setTitle

In addition, Dialog has a bottom-left-slot that displays an EmptyPanel by default but can be used to, for example, display a bread-crumb widget (see AbstractBrowserDialog line 78) or a date-picker (see

SchedulePublishDialog line 64).