uimenu implementation, 2nd version

[Kai Habel]

 Hello,

this is the second version of an uimenu implementation for the fltk 
backend. It should be compatible with matlab in most cases. My first 
implementation was more or less a proof-of-concept, this one is more 
careful designed. (I don't claim to be good C++ programmer, so if you 
think something should be done differently, I am open for suggestions)
This time I have separated the uimenu functions from the plot_window in 
a different class. Keep in mind that not all menu properties in fltk can 
be changed dynamically, therefore I have to rebuild the entrire menu in 
some cases.
I have attached a Changeset relative to the current tip. Changelog, News 
entry and manual entry are not included yet.

Since I have seen some crashes in intermediate versions, it would be 
nice if people regularly bulding octave on Mac and Windows could test 
the changeset. I have attached a simple test file as well.

Kai

Please not that there is already a bug opened for the delayed output of 
a callback function.
https://savannah.gnu.org/bugs/?31121

simple_menu.m
Description: Text Data

Changeset.uimenu_ver2
Description: Text document