Multi client jobs (#15)

* Add API to expose if RenderQueue is available to take new jobs for a given renderer and priority

* Fix issue with calculating Blender percent complete when not starting at 1

* Rename owner / client properties to parent / children

* Add make_ready method to API

* Create and submit subjobs to other servers

* Update make_ready to update children jobs and some misc fixes

* Misc GUI cleanup

lib/server/server_proxy.py

@@ -84,7 +84,7 @@ class RenderServerProxy:
         self.__update_in_background = False
 
     def get_jobs(self, timeout=5, ignore_token=False):
-        if not self.__update_in_background:
+        if not self.__update_in_background or ignore_token:
             self.__update_job_cache(timeout, ignore_token)
         return self.__jobs_cache.copy() if self.__jobs_cache else None
 
@@ -104,6 +104,14 @@ class RenderServerProxy:
         all_data = self.request_data('full_status', timeout=timeout)
         return all_data
 
+    def cancel_job(self, job_id, confirm=False):
+        return self.request_data(f'job/{job_id}/cancel?confirm={confirm}')
+
+    def is_available_for_job(self, renderer, priority=2):
+        request_data = self.request_data(f'is_available_for_job?renderer={renderer}&priority={priority}',
+                                         timeout=1) or {}
+        return request_data.get('is_available', False)
+
     def post_job_to_server(self, input_path, job_list, callback=None):
         # Prepare the form data
         encoder = MultipartEncoder({