Make 'no miniaturizable' window property user-configurable
The flag to not let windows be minimized was already defined in
WM (it's called 'no_miniaturizable') and the minimize functions
respect it. However this flag can not currently be set manually.
As can be read in the NEWS file
"NotMiniaturizable option changed to NoMiniaturizeButton"
it seems that the "non miniaturizable" property was a first-class citizen
in ancient times and probably configurable (those changes do not appear in
the old CVS history).
Let's make this property be user-configurable through the "Advanced Options"
panel.
Signed-off-by: Carlos R. Mafra <crmafra@gmail.com>