Refactor Plugin Loading

[eiennohito]

Fixes #17 Fixes #19 Fixes #20 Fixes #21 Fixes #24

• Plugin loading logic is refactored, no repeated code now
• DSO plugins can now be resolved relative to executable with the syntax: $exe/plugin or relative to the configuration folder $cfg/plugin.
• It is recommended not to specify platform dependent name parts for DSO plugins: plugin instead of libplugin.so. First name will correctly resolve on Windows/Linux/macOS.
• Built-in plugins are moved into the main crate, they can be specified in the configuration by their Java names (but that needs checking).

Main design for the rework of plugin loader

We have core logic of plugin loading in a PluginLoader struct which is generic over implementations of PluginCategory trait.
PluginCategory captures plugin category specific logic: providing specific types, configuration extraction, and instantiation of bundled plugins.

PluginLoader is using fully qualified trait call syntax (see previous paragraph before this link). It is required because type inference does not work if T is not present as one of arguments.

[eiennohito]

Need to finish moving plugins into the main package to fix failing tests.

[eiennohito]

This PR also should be squashed, I think.

[eiennohito]

@mh-northlander ready for review & merging