Use a heap allocated buffer (with lifetime of Aria2 class) instead of
a stack allocated one.
Original fix made by @ZaWertun. Kudos to him.
Fix #kiwix/kiwix-desktop#123, kiwix/kiwix-desktop#513
and kiwix/kiwix-desktop#423
Downloader::startDownload has a new parameter option which is a vector of
pair that represents the options that can be set for adding a uri with aria2
with the function Aria2::addUri.
Aria2::addUri uses this parameter to set the struct of parameters for the
aria2 command
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.