Add ability to rotate placement preview objects

[ampersand38]

When merged this pull request will:

• Enable rotation of placement preview when holding Zeus rotation modifier, default L Shift
• Preview rotation is preserved if Shift is released
• Object can be placed while Shift is held down
• Object rotation matches preview

[mharis001]

Has weird handling when the mouse world position is "past" the object's position (see images).

Images

Placement preview (cursor is below the object in both cases as seen by the icon here):

Normal Zeus rotation:

[mharis001]

I pushed a simple change that should improve the behaviour and cover all cases. Could use a second look.

[ampersand38]

I pushed a simple change that should improve the behaviour and cover all cases. Could use a second look.

You've managed to copy the vanilla zeus rotation behaviour xD I had always wondered why it didn't point directly to the cursor, but felt a little off. It's working from 2D screenpos!