Fix Software becoming unresponsive when downloading screenshots

We now use soup_session_send_and_read_{async,finish} calls which should not block while downloading screenshots.

Note:

We use g_memory_input_stream_new_from_bytes (bytes) rather than gdk_pixbuf_new_from_bytes (bytes), as gdk_pixbuf_new_from_bytes () doesn't auto-detect all image metadata from the provided image in buffer. Auto-detection works only with streams, where GdkPixbuf loaders detect the image type and other image type specific metadata from the stream. So, we convert the GBytes to a GMemoryInputStream.

Closes #2269 (closed)