This functions enable to stop and resume download with aria2.
The Downloader's constructor now checks the paused downloads with the
function "tellWaiting()" to get them at the start of kiwix-desktop.
- "winsock2.h" needs to be included before "windows.h". But if a
compilation unit include "windows.h" and after "networkTools.h", we
fails and it is complicated to handle. The include must not be in the
header but in the cpp
- windows define some ERROR macro. It is a pitty but we cannot use `ERROR`
in our enum.
- If build statically using mingw we need to define `CURL_STATICLIB`
The downloader is using libaria2.
For now, only one download can be run a the time.
A download will start only if (and as soon as) no download is running.