The export functions in mainwindow are getting quite
cumbersome with multiple menu options.
This patch creates a single menu option "Export.." which
allows the user to select a supported format and a filename.
To support this the ridefile reader code needed to be adjusted
to allow registered readers to declare capability to write and
use a consistent (virtual) method to do so.
By modifying the base class for ride file reader we now allow
new readers to register both read and write capability.
