Default mouse buttons of TExamineCamera:

The goal:
"mostly matching shortcuts in other programs" (like Blender) and
"accessible to all users" (which means that e.g. I don't want to use
middle mouse button, as many users have only 2 mouse buttons (or even 1),
besides GNOME hig says users seldom try out other than the 1st button).

Let's check what others use:

Blender:
- rotating: on bmMiddle
- moving left/right/down/up: on Shift + buttonMiddle
- moving closer/further: on Ctrl + buttonMiddle
  (moving down brings closer, up brings further; horizontal move ignored)
Both Shift and Ctrl pressed do nothing.

vrweb:
- rotating: buttonMiddle
- moving closer/further: buttonRight (like in Blender: down closer, up further,
  horizontal doesn't matter)
- moving left/right/down/up: buttonLeft

GIMP normalmap 3d preview:
- rotating: buttonLeft
- moving closer/further: buttonRight (like in Blender: down closer, up further,
  horizontal doesn't matter)
- no moving left/right/down/up.

My thoughts and conclusions:
- rotating seems most natural in Examine mode (that's where this navigation
  mode is the most comfortable), so it should be on buttonLeft (like normalmap)
  with no modifiers (like Blender).
- moving closer/further: 2nd most important action in Examine mode, IMO.
  Goes to buttonRight. For people with 1 mouse button, and for Blender analogy,
  it's also on Ctrl + buttonLeft.
- moving left/right/down/up: buttonMiddle.
  For people with no middle button, and Blender analogy, it's also on
  Shift + buttonLeft.

This achieves a couple of nice goals:
- everything is available with only buttonLeft, for people with 1 mouse button.
- Blender analogy: you can say to just switch "buttonMiddle" to "buttonLeft",
  and it works the same
- OTOH, for people with 3 mouse buttons, that do not catch the fact that
  keyboard modifiers change the navigation, also each mb (without modifier)
  does something different.
