Add keyboard shortcut for hiding all lines in wxt

When plotting many aspects of a dataset in a single graph, it is often convenient to be able to hide all lines and then only selectively enable those that are of interest. This makes 'c' a keyboard shortcut for doing this in wxterminal.

Here, we a case is added to wxtPanel::OnKeyDownChar for handling this. In the long term, a better solution might be to dispatch a command like "hide line 1" to the main gnuplot process, which then triggers a replot, but then there first has to be a way of targeting a command to a specific line.

Patch has been tested against the latest CVS version on Arch Linux x64.