Tcx: fix speed in summary information

When exporting files to Tcx, it's writing an ActivityExtension to the end
of the file with calculated AvgSpeed and MaxSpeed. Unfortunatly it's using
the km/h values - while Tcx is using m/sec. Schema is a bit unclear about
this, as the ActivityExtension uses "double" as type for both values, but
original Tcx v2 Schema only uses m/sec, GTC exports as m/sec and some
other software is expecting m/sec, aswell.
Merge pull request #679 from rclasen/fix-tcx-speed

#678: Tcx: fix speed in summary information
LTM presets UX improvement

Simplified selecting and managing preset charts
in the LTM chart settings.

Need to now move the 'Add' button to the custom
tab, but that is the next pending update.
fix PWX export: handle end of gaps gracefully.

As I got it pwx by itself has no concept of recording intervals. "smart
recording" and other strange data require variable recoring intervals.

On the other hand it's not recording the duration of a sample explicitly.
This means the duration needs to be derived from the previous samples
timestamp (assuming the sample timestamps are referring to the end of the
sampled period).

This diff writes an empty timestamp at the end of each gap so that the
next real sample/ridepoint can calculate the correct duration.

Please note that this may unhide some deficiencies in GCs file reading in
exports.
fix PWX export: always write summarydata

xml schema is requiring summarydata to exist. Otherwise upload to
trainingpeaks fails.

Block was omitted when no intervals were defined.
PWX export: keep timestamp + distance precision

Pwx does support for arbitrary sample lengths. No need to chop off
timestamp precision and introduce bad data + jitter.

Instead we should look at fixing data import and consider resampling on
read.

Distance is defined as double, aswell - so, let's keep the precision,
aswell.
PWX: read/write temperature

added temperature to read/write
use pwx for Trainingstagebuch upload

as current tcx writer currently doesn't support laps (and tcx doesn't
allow non-linear/overlapping intervals, at all) this patch makes GC upload
exercises as pwx.
Merge pull request #682 from rclasen/fix-pwx

several pwx fixes / improvements
Merge pull request #683 from rclasen/ttb-pwx

use pwx for Trainingstagebuch upload