Add a zoom selector on status bar

A single click on zoom indicator on status bar opens a zoom selector.
It seems quite natural to click on the indicator to change the corresponding
value. As zoom level is displayed on statusbar, a click on statusbar should
open a zoom selector.

This patch is inspired by ac4478f4c84881436cf9b25a0fa9dc30f889db63
"Add a zoom combo box into the toolbar".

Signed-off-by: Guilhem Bonnefille <guilhem.bonnefille@gmail.com>
Remove zoom combobox in toolbar

Since it is possible to change the zoom level from the status bar,
there is no more need for an other zoom level selector.
Furthermore, it is always better to have a GUI as clean as possible.

This patch remove the combo box allowing to select the zoom level
from the tool bar.

This patch also refactor the zoom levels related list.

Signed-off-by: Guilhem Bonnefille <guilhem.bonnefille@gmail.com>
Ensure tracks coloured according to the draw mode.
Add simple dialog to show map tile information.

Includes source of tile data and filename+timestamp of the cached tile
on disk.