Convert PluginsManager into singleton
Clean up plugins, use .desktop-based search

The PluginsInterface is gone now, using only AbstractPlugin as a base
for all plugins.

Instead of probing each library for some hard-coded symbols, we use
.desktop files installed in /usr/share/tepsonic/plugins that contain
all metadata about each plugin.

Some more cleanup may be done in future, I'm still not 100% happy with
the entire plugins framework
Thread pools are fine

Apparently  I was not /that/ stupid back then...
Update copyright headers and #includes