We move the drive selector warning to the flash step, and concatenate
warning messages when more than one needs to be displayed at once.
Change-Type: patch
Changelog-Entry: Move the drive selector warning dialog to the flash step.
* feat(GUI): add app to gui folder structure
We add a `lib/gui/app/` folder to help transition to Webpack usage.
Change-Type: patch
Changelog-Entry: Add `lib/gui/app` folder to ease into Webpack usage.