Download and extract packages in a separate thread + installation queue

[vgmoose]

Upon initiating a download for a package, the download/extraction progress status and bar should appear in a bar along the bottom, and allow the store to continue to be used to queue up more packages to be added to the bottom bar. Would also need #24 to be done properly (should display 1/2 Downloading, 2/2 Extracting)

[vgmoose]

Also, the button should become disabled and just read "Installing...." / "Queued..." / "Removing..." depending on what it's waiting on.

[vgmoose]

std::thread is available in libnx now!

[vgmoose]

After playing with threads for a bit, and with issues on various platforms, I'm deciding it's best to keep our single threaded approach and come at managing downloads from the other end– the same way progress bars currently work via callbacks, but also process input at the same time to effectively allow multitasking while downloads occur.

As such, the installation queue talked about in this issue can be resolved with fortheusers/libget#14