This PR change the way the application links with the plugins and the intermediary library (libmellowplayer). Everything will now get linked statically

There are a few advantages to use static linking over dynamic linking:

• it's infinitely easier to deploy (especially on mac and windows)
• it's also easier to document, there is one single procedure for adding a plugin and contributing to the project.
• there is no need to package libraries, headers, pkgconfigs,...
• this ensure a review of the plugin by the upstream team before a plugin is released in the wild,...

Basically the idea is to keep the system as simple as possible for the developers (following the KISS principles).