These points can be used for constraining the width of the text
(or to the width of the text).
The main parts of the commit are:
* TtfFont is restructured to be able to return the aspect ratio
for a given string.
* This aspect ratio is written to the savefile, such that even if
the font is missing, the sketch would still be solved correctly.
* The two additional points are constrained via perpendicularly
to the two main points (which form a v vector).
The compatibility features are as follows:
* When the font is missing in old files, 1:1 aspect ratio is used,
which works for the replacement symbol anyhow.
* When the two additional points are missing in old files, their
would-be positions are calculated and they are moved there,
avoiding 'jumping' of underconstrained sketches.