[sigc] conditions in libsigc

[Paul Pogonyshev <pogonyshev gmx net>]

Hi,

I would like to propose conditions in libsigc.  A condition is basically
a boolean vairable plus signal which is emitted each time the variable
state is changed (from true to false or vice versa.)  They are useful in
e.g. GUI design because you can specify a boolean expression, e.g.
sensitivity or visibility, only once and not track when its state changes,
which can be difficult at times.  In other words, the condition itself
and its state tracking are no longer separated and chances of programming
error are significantly smaller.

I volunteer to implement this if you are interested.

Paul