persistent listeners

[Konstantinos Poulios]

In the new implementation of subplot we have the problem that the
listeners that we use for the synchronization of subplots are deleted
once something is plotted on a subplot.

The attached patches implement persistent listeners, i.e.:

1. listeners added with:
addlistener (handle, propertyname, cbfunction, "persistent")
will not be removed by the c++ method "remove_all_listeners", nor by a
call to delete_listener() with empty arguments list.

2. however, explicitly removing a persistent listener from the frontend with:
dellistener (handle, propertyname, cbfunction)
should still work.

Please take a look at the patches so that I can push them if it is ok,
so that we can make some progress with the subplot fixes.

Kostas

persistent-listeners-1.patch
Description: Text Data

persistent-listeners-2.patch
Description: Text Data