#471 before loading a GUI plugin, check its not already loaded

[Hans-Christoph Steiner]

only load GUI plugins with the same name once, to prevent mayhem

The actual file name of the plugin is stored in a list. Whenever a plugin
is being loaded, first pd-gui checks whether that file name is listed as
loaded. If its loaded already, pd-gui prints a message then ignores it

[Hans-Christoph Steiner]

patch to apply with 'git am'

[Miller Puckette]

Mayhem you say? we needs more of that, what?

OK for 0.44