Introduce a proper plugin system

[Malcohol]

A proper interface for dynamically loading plugins.

Plugins are not expected to work across versions or different build environments, since deep C++ dependencies are expected to exist between plugin and host. However, some flexibility can exist and I've tried to make the plugin handshake somewhat robust, at least to the extent of issuing informative errors when possible.

In this initial commit of plugin functionality, all plugins are automatically loaded during start up.

Changes:

• BuildCompatibility testing
• A Version class and application version settings.
• A cmake function for defining a library as a plugin: babelwires_add_plugin
• Plugin discovery and loading logic
• The libs that should be plugins are now dynamically loaded using the plugin system.