wslview: make configuration more robust

[benlorenz]

fixes #417

This will print the following once after autoconfiguration:

┌ Warning: /usr/bin/wslview not found, to use native viewers for
│          visualization on Windows (with WSL) please install wslview
│          and run `Polymake.configure_wslview()` afterwards.
└ @ Polymake ~/Polymake.jl/src/visual.jl:150

Subsequent starts of Polymake.jl will not print this warning, until it is updated, loaded from a different source directory, or the scratchspace is cleared.