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/icons/thin/downloading.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"><path d="M12 21l-8-9h6v-12h4v12h6l-8 9zm9-1v2h-18v-2h-2v4h22v-4h-2z"/></svg>

src/icons/thin/uploading.svg

@@ -1,3 +1 @@
-<svg height="24" viewBox="0 0 24 24" width="24" xmlns="http://www.w3.org/2000/svg">
-    <path d="m16 16h-3v5h-2v-5h-3l4-4zm3.479-5.908c-.212-3.951-3.473-7.092-7.479-7.092s-7.267 3.141-7.479 7.092c-2.57.463-4.521 2.706-4.521 5.408 0 3.037 2.463 5.5 5.5 5.5h3.5v-2h-3.5c-1.93 0-3.5-1.57-3.5-3.5 0-2.797 2.479-3.833 4.433-3.72-.167-4.218 2.208-6.78 5.567-6.78 3.453 0 5.891 2.797 5.567 6.78 1.745-.046 4.433.751 4.433 3.72 0 1.93-1.57 3.5-3.5 3.5h-3.5v2h3.5c3.037 0 5.5-2.463 5.5-5.5 0-2.702-1.951-4.945-4.521-5.408z"/>
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"><path d="M10 9h-6l8-9 8 9h-6v11h-4v-11zm11 11v2h-18v-2h-2v4h22v-4h-2z"/></svg>