Make 'add content pack' dialog more robust to missing images
Copied changes from main branch (CL 2400859) to hopefully ease merging
#codereview bruce.nesbit, frank.fella
[CL 2419717 by Ben Marsh in Main branch]
Added display of asset/class info used to Add Content dialog
Fixed several cut/pasted comments regarding UMG styles that referred to shooter game
[CL 2392502 by bruce nesbit in Main branch]
+ Refresh feature packs automatically when they change on disk.
+ Remove drag and drop and batch install, and replace it with a single button for installing in the details section.
+ Don't allow multiple copies of the dialog to be opened.
+ Fix a missing brush visible when resizing the dialog.
+ Fix an issue where brushes in the dialog could get deleted resulting in white images.
[CL 2385434 by Frank Fella in Main branch]
