Launcher: Add a way to automatically download the game from Steam
"These alternative UI mockups":https://forum.openmw.org/viewtopic.php?f=20&t=4372 for the Launcher were well-received on the forum. One part of this was adding the ability to automatically download the game from Steam. Basically, it would be automating "these instructions":https://wiki.openmw.org/index.php?title=Installation_Instructions#Steam, specifically the creation of appmanifest_22320.acf so that the app can be downloaded next time Steam is opened.
The attached mockups show the part of the UI that would be changed.
(RM-3984 from redmine: created on 2017-08-03 by Will Herrmann, )