Gutter: improve documentation

The FIXMEs will be fixed progressively, as I better understand the code.
Deprecate the Gutter:xpad and Gutter:ypad properties

Each GutterRenderer can have their own padding. A renderer better knows
what padding to use. So the global padding is not really useful.

Moreover, the global ypad was not implemented, and the global xpad was
broken. Now the implementation of the global xpad has been removed. They
are now just "dumb" properties.

These properties were used nowhere in gedit, gedit-plugins and
gedit-code-assistance.
Deprecate gtk_source_gutter_get_window()

It's a small convenience function, used nowhere in gedit or
gedit-plugins. All the information is available from the outside for
reimplementing the function.
Gutter: improve doc