gui/macapp: slew of code cleanup; unmount filesystems on quit

gui/macapp: slew of code cleanup; unmount filesystems on quit

a handful of code cleanup, renaming and refactoring.  basically consolidating
'application logic' (mount/unmount fs) into the 'MacGuiApp' class (the wx.App)
and cleaning up various scoping things around that.  renamed all references to
'app' to refer more clearly to the 'AppContainer' or to the guiapp.

globally renamed basedir -> nodedir

also made the guiapp keep a note of each filesystem it mounts, and unmount
them upon 'quit' so as to cleanup the user's environment before the tahoe node
vanishes from out underneath the orphaned tahoe fuse processes