tiling: change in config storage

Change from storing the layout in groups [output][virtualdesktop]tiles to [virtualdesktop][output]tiles since this change wasn't shipped yet, is better this way for 6.3 as in 6.4 we want to refactor the tile hyerarchy in a single big tree which will have

globalroot |-desktop1 | |-output1 | | |-rootTile | | | |-tile1 | | | |- ... | |-output2 | | |-rootTile | | | |-... |-desktop2 | |-output1 | | |-rootTile | | | |-tile1 | | | |- ... | |-output2 | | |-rootTile | | | |-... ...

This config format better agrees wit hthe new format it will have in memory