vlapi: readdir() change return type to return read data length

This is necessary since we need to pass the length of the read data to
conv_dir() in a later patch.
vlconv: fix conv_dir()

Testing has shown, that the function conv_dir() as it is does
not contain a valid condition to end the parsing loop of data
in the buffer.
Reasons might be a change in the api in the past, or (less likely)
the Volkslogger used for testing having a different protocol than
the api expects.
Some hints in the code point to a not finished work in the past
which left the code not working.
The exact details of the volkslogger protocol are not known,
so it is not possible to tell why this is the case.
The working solution to the problem is to pass the number of chars
in the buffer to the function and stop parsing after all data was
processed.
vlapi:Replace new/delete by std::vector

The line with delete[] caused a segmentation fault.
The new/delete[] was replaced by switching to
std::vector.
This allows us to do some more cleanup:
-DIRECTORY structure obsolete.
-Since we have conv_dir() now using std::vector.
The first "count only" call of the function is
no longer necessary.
The original use was to determine the count of
entries so the dynamic array could be initialized
with the correct size. std::vector makes that
obsolete.
-int number_of_flights used in conv_dir() is
no longer necessary
conv_dir: make caller responsible for clearing the data vector
Volkslogger igc download: Activate download feature in DeviceRegister

Modify Register.cpp to make xcs aware that Volkslogger supports flight
download.
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.
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 %
vlconv: Make parsing loops check env.IsCancelled()

Make it possible to abort during parsing loops.
If aborted delete the written file.