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.
GTC will export all rides as a single TCX file so they
can be imported en-masse into another application.
We did not support >1 rides in a single ride file. This
patch adds support for reading multiple rides (if the ride
file reader supports it).
The ride import wizard will now extract and parse files from
a GTC export. Many thanks to Damien for writing the TCX file writer.
Fixes#371.
