For mobile, save log downloads next to where everything else is saved instead of straight into the home directory (which is anybody's guess when it comes to mobile platforms).

Adding missing qgcView to QGCMapPolygonVisuals instance in SurveyMapVisual.qml

The timer was only started in _logData(), which was called after receiving
the first chunk of data (and only if that was successful), which means
if the first request failed or timed out, the UI would never switch to
a timed out state.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

On PX4, under certain circumstances and in rare cases, there can be log
files with 0 size. In that case QGC ignored the entry and tried to
re-request the entry indefinitely.

1. PX4Log parser is split off into its own class
2. New ULog parser implemented

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>