[orca-list] Small status update for plugin system based orca

[Linux A11y <chrys linux-a11y org>]

Howdy List,

A small update of my current work on an plugin system based orca. 

- i merged in all of Joanies work from master (till 1:00 PM UTC June 17 2021). So the plugin system branch is 
on pair with orca master.

- i created an SignalManager. This component is, like the name says, used for register and emit signals or 
check for its existence. So we can define a set of signals what plugins can connect to or plugins can 
register and emit signals by there own.
- dynamic API manager: well sounds mystic, but the idea behind is that plugins can register own API, objects, 
functions, variables and all kind of that. So others can consume those and/ or  extend its functionality 
based on it

Both objects live in our shiny new orca class.  The orca class should hold all basic components and all 
needed API, as goal, all orca code should in an roof or main class. So we can easy share its API with plugins 
or let them extend it.
I also placed the scriptmanager-, eventmanager-, settings manager classes there (also if they should be 
reworked to fit better in shape) and the plugin system manager. So the way to use ocas API will just happen 
starting from that class containing all API, settings and states.

- i just started implementing an very basic plugin manager. But this is not useable yet. I will inform as 
soon as it is ready for testing.

- i also started with an base class for orca plugins, but here i have some trouble with get mixed plugin 
objects in libpeas ( they all register just fine, but deregister all as clipboard plugin. Not sure what mix 
it up, but i will figure anytime lol)

- we already have some early adopters for testing to warranty we don't break anything. Thanks to Patrick and 
storm here for much needed testing!
 
Current goal is just to bring an basic plugin structure in without break anything. No new settings, no real 
new functionality. All should just work as it is.

Cheers chrys