Create a command palette with hotkeys for Kibana

[timroes]

We should build a command palette, that will open anywhere in Kibana with a specific shortcut. You can search in this Command Palette for specific actions you can do on the current screen and also some global navigation links to jump to other applications/screens within Kibana.

I've created a small proof of concept in my commandpalette-poc branch, and demonstrate the usage in the following screencast:

Which each command you register, you should be able to specify a shortcut for that action. The command palette will take care of executing that command when you press it, and also showing the shortcuts, so users can learn them:

For proper accessibility (and thus usability) every feature should be able to be used by mouse and keyboard, meaning we should never make the command palette the single place to access a specific functionality, but always also provide a UI for it.

There are a few things to think about how these should work to provide the best usability:

• We would like to register e.g. all saved dashboards in there to jump to. Since this can be a lot, you most likely don't want them to show up all the time. So a concept of nested menu would be nice, e.g. searching for "Dashboards" hit enter and the list of dashboards show up again. Since we want to make it more efficient for pro-users to work with this, we could consider a prefix, like typing db and in this moment the palette will start searching in all dashboards.
• If we register those commands from within the applications, e.g. timelion register it's own link to jump towards it, those won't be loaded until you switch to the timelion application. We need a concept to register navigational links without the application being loaded.
• We should integrate with existing features like the feature directory on the home page, so we don't need to register all links twice. This might even solve the above issue.
• Consider whether we want to rewrite shortcuts with Ctrl to Cmd for OS X users.

This issue would also fix #4539.

Relevant WCAG Criteria: 2.4.5 Multiple Ways - Level AA