[TASK] Adapt gridelements support to newest gridelements

This change avoids setting colPos values when a gridelements drop zone value is detected (a relative "uid" containing an `x` character). Rather than attempting to set a colPos value this value is left untouched. What remains is clearing the `tx_flux_parent` and `tx_flux_column` fields to clear any relationship set by Flux, leaving it up to gridelements to handle the element from then on.
[BUGFIX] Do not return fields from a Sheet if field is inside Container

Close: #577