Rulers in <richtext>

Legend (n and m here are some decimal digits):

“healthy” columns:

caption	meaning
nn	fixed column, width of the column in pixels
nn%	percentable column, width of the column in percents
<nn>	auto column, nn here actual width in pixels
>nn<	auto column reached its' minimum

“problem” columns:

caption	meaning
nn(mm)	fixed column, declared as nnwidth but resized to mm pixels.
nn%	percentable column but not enough space to render it using nn% value.