Action bar greeting/farewell

**Is your feature request related to a problem? Please describe.**

There are flags for greeting/farewell that appears in the chat or as a title. Both types of flags have their disadvantages: the chat one spams the chat if user is entering/leaving regions many times, and the title one is too big and prominent for small informative greetings.

These are the flags I'm talking about:

> greeting | string | The message that appears in chat upon entering the region
> greeting-title | string | The title that appears upon entering the region. Including a newline (\n) will send a subtitle.
> farewell | string | The message that appears in chat upon leaving the region
> farewell-title | string | The title that appears upon leaving the region. Including a newline (\n) will send a subtitle.

**Describe the solution you'd like**

I propose adding two new flags, maybe named "greeting-bar" and "farewell-bar", that would show an action bar when entering/leaving the region. It'd work like the command:

` /title @s actionbar "test greeting"`

**Describe alternatives you've considered**

I believe there aren't any plugins with flags like that. It could be done with console-command-on-entry/console-command-on-exit in WorldGuard Extra Flags, but that seems like an overcomplicated solution for a simple feature like that.