Add Download Time Prediction

Issue: There's no way of knowing that CV-HDRI and other large libs are slow to download instead of broken.

Suggested solution: * List file-size of plugins/tools. * Show download progress as a string as in Google Chrome: "32 Seconds Remaining -- 247 KB of 1.1 MB" * Prioritize the installation of smaller plugins/items first. * Add a "Cancel" button to the download/install page.