Qt: Load and save settings

Everything *should* work, from QSettings (saved settings) to
QTouTvConfig to the preference dialog.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Qt: Reorder initialization

...so that config is loaded and broadcasted before we start download
stuff.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
TransportJson: add set_http_proxy method

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Merge branch 'v2-base' into v2-qt-dev
Qt: display default values in interface
Qt: config: handle key not in defaults

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>