Having different include files that are needed (and which are
inter-dependent) to create your own plugin, makes things harder than it
needs to be.
Therefore, all plugin header files (those for the "new" (1.4) plugin
framework anyway) have been combined into one header file. Thus,
developers now only have to download a single file and include that
instead of having to figure out what files to download and what to
include where.
Taking the chance, the version number has been removed from the header
file's name. This allows one to track changes made to the API via git
(which is not quite as easy if you create a new file every time you make
a change).
Previously the plugin installer attempted to select the correct plugin
binary by its file extension but it turns out that this is not a unique
choice (e.g. .so is supported on macOS and on Linux).
Therefore this commit introduces a mandatory manifest file that is to be
present in plugin bundles that contains the mapping for which binary to
use on which OS and for which architecture.
Because a plugin bundle now has to follow such a strict format, it is
now also mandatory for the bundle to have the file extension
.mumble_plugin (previously .zip was allowed as well).
