Resolve conflicts between invisible and deleted primitives automatically

[Gubaer]

• server has "invisible" element and local element is "deleted" -> "purge" it
• server has "invisible" element and local element is "unmodified" -> "purge" it

where

• "invisible" - visible=false
• "deleted" - deleted=true
• "unmodified" - modified=false
• "purge" - remove all references to the element in the dataset and either delete the primitive physically from the dataset or set "visible=false" (not sure whether the later would work in the local dataset, must of the code just checks for "deleted" not for "visible"; currently the code always phyiscally removes primitives from the dataset if it "purges")

See ​thread on josm-dev.

[Gubaer]

(In [2198]) fixed #3249: Resolve conflicts between invisible and deleted primitives automatically