Fix problems with libpng
Re #1409 and #1444:
* added support for two FLVSS
* added gtests for FLVSS
Fixes #1478
Fixes #1481
Fixes #1475
Fixes #1478
Merge pull request #1477 from opentx/projectkk2glider/cells_gtests

Tests and support for second FLVSS
Merge origin/next into romolo/modeledit_layout
Remove useless code