wx GUI: Improve code
wx GUI: Don't process Tab in the editor text area by default

Addresses #352
wx GUI: Temporary fix for #331
wx GUI: Temporarily work around bug #332
wx GUI: Don't Tab-traverse editor plugins' widgets when panels are collapsed

Fixes #353
wx GUI: Allow expanding/collapsing editor plugin panels with Space or Enter

Fixes #333
wx GUI: Highlight plugin panel bars when focused