Merge reduceMemoryUsageForPreviewPanelsAndQuicklook branch into v3.1
See comments on this commit: https://github.com/gnachman/iTerm2/commit/833832ad766cda501cb4b711745b6d1ce31e61e2#commitcomment-17657678
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
See comments on this commit: https://github.com/gnachman/iTerm2/commit/833832ad766cda501cb4b711745b6d1ce31e61e2#commitcomment-17657678