Fix dataloss when flex inset is undefined.

Fix dataloss when flex inset is undefined.

Regression at cfeddb929. If a flex inset has no layout upon saving (e.g. if a
module has been deleted) then its name became lost. This checks whether the name
resolution, introduced with the ObsoletedBy tag, comes back empty-handed (which
it will if the layout is not defined). In this case, we do as was done before
cfeddb929.

In addition, the use of support::token to strip "Flex:" off the beginning of the
name introduces a regression if somebody used a name containing ":". This
replaces it with support::split.