Support declarative keystrokes in Visual Editing

HTML field configuration can define keyboard shortcuts, a.k.a. "keystrokes". The default way of configuring this is 'numeric keystrokes', which basically adds ASCII codes of numbers and special keys like Ctrl and Shift. Since that's not very nice for Hippo's repository-based JSON configuration, Hippo also supports 'declarative keystrokes', which allow you to define keystrokes via named keys. The difference is documented at https://www.onehippo.org/library/concepts/document-types/html-fields/ckeditor-keyboard-shortcuts.html.

The CMS parses declarative keystrokes in HTML field configuration and converts them to numeric ones. The visual editing backend should do the same, which means the parsing code should move to a shared location so it can be used by both the CMS and the Visual Editing backend. It uses the org.json library for parsing JSON, which should be converted to Jackson since org.json has become a do-no-use library.

As a result, visual editing should also support the default keystrokes 'Ctrl + M' (to maximize the editor) and 'Alt + B' (to toggle 'show borders of block elements').