fix #23: jumping depth slider on mouse wheel

This targets bug report #23. With a modified wheelEvent handler the
depth slider can be adjusted in single steps when turning the mouse
wheel.

In my opinion it is also more natural to have something that controls
depth from high to low to be arranged in vertical direction.
fix for #15: adding keyboard control to depth (Y)

Added keyboard control for depth (Y) slider. When MapView widget is
selected (e.g. after moving zooming) there are now keys to
increase/decrase depth level: HOME END + -
While holding down SHIFT it is also possible to use the mouse wheel to
change depth instead of zoom.
simplified depth control with mouse wheel

Previos code was too complicated, as SHIFT status is already present in
wheelEvent.
Merge remote-tracking branch 'upstream/master' into ControllingTheDepthSlider
Merge remote-tracking branch 'upstream/master' into ControllingTheDepthSlider
added configuration for vertical/horizontal depth slider

Now the setting dialog box contains a checkbox to select if the
traditional horizontal depth slider shall be used or the new proposed
vertical one.
I decided that the change will take place only after a Minutor restart,
as it would be complicated to change the layout after setup.
The setting dialog is also converted to a Qt Form, which makes it easier
to add changes.
Merge pull request #40 from EtlamGit/ControllingTheDepthSlider

Controlling the depth slider with mouse wheel and keyboard