This commit introduces a new plugin framework into the codebase of the
Mumble client. Note that "plugin" here really refers to a (more or less)
general purpose plugin and is therefore not to be confused with the
previously available positional data plugins (only responsible for
fetching positional data from a running game and passing that to
Mumble).
The plugin interface is written in C, removing the compiler-dependence
the old "plugins" had. Instead plugins can now be written in an
arbitrary language as long as that language is capable of being compiled
into a shared library and also being capable of being C-compatible.
As already indicated a plugin is essentially a shared library that
provides certain functions that allow Mumble to interface with it.
Inside Mumble the so-called PluginManager is responsible for managing
the plugins and relaying events to the respective callbacks. Plugins
themselves can also interact with Mumble on their own initiative by
using the provided API functions.
Fixes #2455
Fixes #2148
Fixes #1594
Fixes #2051
Fixes #3742
Fixes #4575
Fixes #4751
