we don't need a build breakage on the first deprecated item, thanks

- use *.plugin extension instead of *.pluma-plugin
- use "Plugin" section name instead of "Pluma Plugin"

mostly adapted from:
https://git.gnome.org/browse/gedit/commit/?id=1d56b827f14096891bb03dbd5e8211fdad1331ad
https://git.gnome.org/browse/gedit/commit/?id=7c22769d73f756b9d1388febd147c9d267ed54cb

There are three options for autocheck when a doc is loaded:

Always autocheck, Never autocheck and Remember the autocheck setting for each document

The chosen option is stored in the gsettings schema.
The default setting is to remember by document as this is what Pluma did previously.