Scene navigation improvements in the editor

Hello,

I've made some changes which make it easier to move around the scene when using the editor.  The patch makes E and Q move up and down, as in Unity and UDK.  Following Blender, it makes the numeric keypad 1, 3, and 7 keys select front, side and top views.  (Again as in Blender, control with those keys selects the opposite view—back, left and bottom.)

I also modified the code which tracks the camera's pitch and yaw, so it doesn't gimbal lock when you are facing straight up or straight down.  Of course there is a lot of personal preference in the way mouse look works, so I'll understand if you decide you are happy with the way it is now. :)

Thank you for your time developing Urho3D, and if you'd like to include these changes, please help yourself.  I have only just started experimenting with Urho3D so it's possible I've duplicated some existing functionality, or implemented this in a way that isn't sensible.

Pete