Add support for discovering and executing quick actions from the editing area using the keyboard

The goal is to:

• allow simple users to discover and execute quick actions from the editing area using the keyboard; many of the editing features already have a keyboard shortcut but it's hard to remember all these shortcuts; it would be simpler / faster to have a single shortcut, say / (slash) that allows the user to search / filter quick actions and to execute them directly from the editing area; a quick action could be to apply some formatting, to insert an image, a link, a table or a macro, in general any feature that is supported by the editor
• allow developers to implement quick actions in editor plugins
• allow administrators to configure the available quick actions (include / exclude list)

This issue is not about implementing any specific quick action but rather about implementing the framework that supports the implementation, configuration, discovery and execution of quick actions. Basically the list of quick actions should be dynamic (similar to the toolbar, without any hard-coded item).

We'll open separate issues for implementing specific quick actions, e.g. quick actions for:

• formatting
• inserting links
• inserting / uploading images
• inserting tables
• inserting macros (XWIKI-19982)
• etc.