vlapi: Implement Bulkrate

Make Bulkrate configurable from UI.
This Bulkrate applies only to the download of
the actual flight log.
The download of the flight list (called DIRECTORY)
and the signature have to be carried out with
standard IO Rate 9600 baud.
9600 baud is also the minimum bulkrate supported.
vlapi: Enhance progress bar functionality

Create a cyling progress bar.
Since the amount of data is not known before
transmission it is not possible to create
a single progress bar which would run from 0
to 100 %
vlapi: Generate user message to wait for Volkslogger

To make the user aware of the Volkslogger taking time
to calculate file security a message is generated.
Volkslogger igc download: Activate download feature in DeviceRegister

Modify Register.cpp to make xcs aware that Volkslogger supports flight
download.