Add Support for Update Plugin by Upload

Sometimes it becomes necessary to upload a new version of a plugin that may not be presently available or accessible on the WP plugin directory, but manually uploading a plugin is problematic because WP generates an error if the plugin directory already exists. The current process to manually update a plugin in the WP admin panel is to 1) deactivate the currently installed plugin. 2) uninstall the plugin. 3) upload the new plugin replacement. 4) activate the plugin. Uninstalling the plugin can be particularly disastrous if it deletes records with user-defined data that the reinstalled plugin will need to properly operate. Instead of carte-blanche generating an error, if the plugin directory already exists (there is a good use case to keep it in some circumstances), it would be more helpful and time saving to simply prompt the user of the potential conflict, before proceeding. An alternative would be to have an additional button for “Upload Update.”