Update libdecor source link in FAQ and package name to dev package

[JJRcop]

Update link from https://gitlab.gnome.org/jadahl/libdecor to https://gitlab.freedesktop.org/libdecor/libdecor
Change package name from libdecor to libdecor-0-dev to help those that may install the non-devel dependency.

[netboy3]

Also explicitly include libdecor-0-0 to eliminate reliance on the Debian dependency relationship between them.

Why? I thought we only list the necessary packages and not include their dependency path. All library dev packages will always have a dependency on their runtime shared library package. If we decide to change the process to "eliminate reliance on the Debian dependency relationship", we have to add the non-dev packages throughout the documentation as they are all missing.

[JJRcop]

Why? I thought we only list the necessary packages and not include their dependency path. All library dev packages will always have a dependency on their runtime shared library package.

Of course! It slipped my mind at the time. Maybe not a great thing to happen to the documentation guy, but thanks for the reminder.