Remove pluma-marshal.

[mbkma]

Instead use the generic marshaller. Less files, less to maintain.

See:

• https://developer.gnome.org/gobject/stable/gobject-Signals.html#g-signal-new
• http://sourceware.org/libffi/
• https://gitlab.gnome.org/GNOME/gedit/-/commit/373adb7d041b0559953d0ac8cf2213ecc44db956

[raveit65]

@mbkma Can you please rebase with master to fix archlinux build issue at travis?