core, chat: upload plugin system: - extented plugin system so it's not only used with main widget. It is also used for upload widgets and can be extended more - plugin file name is used to detect the type: plugin_wid_* for main widgets, plugin_upload_* for upload widget plugins - a new UploadMenu class allows to easily add an upload button which will use loaded plugins - plugin_info can now specify a list of allowed platforms in "platforms" key - file upload in chat has been moved to a plugin