Text extent implemented (partially)

[Michael Goffioul]

FYI,

I've pushed the following changeset:

http://hg.savannah.gnu.org/hgweb/octave/rev/9f2bf537a651

This implements text extent property, at least the width and
height parts. The left and bottom are always set to the lower
left corner of the axes box. It also takes into account the
rotation of the text, but only multiples of 90 (these are the
only rotations supported by the renderer anyway).

It also implements correct updaters for the position and
extent properties (for text objects), so you can basically
get the text size in pixels.

I think this should be ok to start looking at the legend stuff.

Note that I won't have the time in the coming days to continue
the implementation, so if anyone want to take over, be my guest.

Michael.