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".

[dieterdreist]

This is not working (without modifications)! The Problem arises on Segment 3: this must be aligned with node 1 (in case of a rectangle), otherwise the angle will not be correct. I am also proposing a concept like this but with one extension: there must be some "tracking" like in AutoCAD-tracking involved. In addition I would like to see a freely configurable angle, not just 45 degr. because there are also buildings based on 60, 30, 120 and other angles (this should be no problem though).

If you are not familiar with autocad-tracking: I made a screencast and sent it to t.wendorff. Maybe he still has it (unfortunately I don't).