Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.

I took the logo_square from:
https://github.com/flutter/identity-assets/tree/master/assets/logo_square
Opened it in preview, gave it a white background
and then dropped it in the icon tool to create
these updated icons.

We may want to use offwhite like we did for Android
but this is sufficient for my demo today.

@hansmuller

We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.

Initial version of complex layout performance test