Add visible indicator when downloading files

Downloading file messages will now show a transfer control above the
composer, similar to uploads. Measuring the progress or pausing the
operation is not possible yet.

src/backend/utils.py

@@ -38,8 +38,10 @@ from .pcn.section import Section
 
 if sys.version_info >= (3, 7):
     from contextlib import asynccontextmanager
+    current_task = asyncio.current_task
 else:
     from async_generator import asynccontextmanager
+    current_task = asyncio.Task.current_task
 
 Size       = Tuple[int, int]
 BytesOrPIL = Union[bytes, PILImage.Image]