* 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
Move System.Drawing usage out of Core to WinForms, as System.Drawing is
not in .NET Core/Standard. Simple methods to return resource name
strings have been added instead.