* delete received items with the delete key
mass selection of received items for mass deletion as well
improved index selection on deletion
* tab indexing for the form
* Move pkhex.drawing calls out of form constructors
designer doesn't like it; if Main can't be designed then there's some
call that it can't process for whatever reason
* Add netcore3 targeting
Closes#2198 by adding a netcore3 build option
Closes#2391 by retaining designer functionality and net46 builds
