Add orthogonal drawing mode

[stoecker]

Much like drawing mode, but with some changes.

• Only for drawing closed ways
• first way segment gives direction. All following segments muste be in steps of 45 degree related to first segment.
• SHIFT-keypress allows unaligned way segments (for non orthogonal parts like roundings)
• drawing is finished when clicking last node again.
• aborting drawing removes entire object, not only the last segment.

Optionally support tagging new object as "building=yes".