Blender Git Loki

Make options for background gradient more organized.

This was difficult to do because we group theme colours and display them
together in user preferences. To make the background options more
presentable and keep them grouped and separate, I needed to group the
two gradient colours somehow. I added a separate ThemeSpaceGradient RNA
struct as opposed to ThemeSpaceGeneric. This struct is the same as
ThemeSpaceGeneric but it lacks the window background option (which does
nothing now) and includes the UiGradient struct which now has both
gradient colours. I modified the clear functions to use a new high
colour from the gradient. Now all options appear grouped and any other
editor that may use a gradient for the window background may do so.

Also corrected incorrect MAIN_VERSION_ATLEAST macro, it would not detect
versions correctly