ENH: Improved crosshair usability, added display in 3D view

Crosshair had several problems/limitations:

1. Crosshair position was often inconsistent in different slice views (for example: when in navigation view, crosshair was always displayed in all slice views because otherwise the user could not drag the crosshair to a new position; but this way when the user changed slices then the crosshair showed up at different RAS positions in each slice view)
2. Navigation mode was very inconvenient: to jump all slices to a specific RAS position the user had to position the mouse to the previous crosshair and drag it to the new position (it was too easy to miss the small navigation area in the middle and when it was missed, brightness/contrast of the image was accidentally modified)
3. By default (if the user just toggled the crosshair button on the toolbar), crosshair was only visible in the current slice view, which was useless (the crosshair just followed the mouse in the current view)
4. Crosshair position was not visible in 3D view

Solution implemented:

1. Only show crosshair in the correct position in all views. If crosshair position is not in the current slice then hide the crosshair from current slice.
2. Crosshair is now moved by holding down shift + moving mouse. No need to grab the previous crosshair position. Navigation mode is replaced by "jump slices" option (enabled by default), which controls if slice views should jump to current crosshair position. If disabled then shift + moving mouse only moves the crosshair.
3. By default slices jump to current crosshair position, so crosshair is visible in all views. User just has to keep Shift key depressed (added hint to tooltip)
4. Implemented display of crosshair in 3D view.