#509 prevent recursive loading of gui-plugins

[IOhannes m zmölnig]

if a gui-plugin loads other plugin, we might easily encounter a recursion (where the plugin tries to load itself).
while the current gui-plugin loader mechanism tries to prevent re-loading of the "same" plugin (based on the filename of the plugin), it doesn't catch recursive loading.
the attached patch fixes this, by adding the to-be-loaded plugin to the "::loaded_plugins" list, then tries to load it and removes it from the list if the loading fails
(rather than adding the plugin to the list after the loading succeeded)

[IOhannes m zmölnig]

apply with "git am ..." (todays git/master)

[IOhannes m zmölnig]

forgot: the patch also makes "load_plugin_script" return '1' if the plugin script has been successfully loaded, or '0' otherwise.

[Hans-Christoph Steiner]

I haven't tested this, but the rationale and the patch make sense to me.