Preliminary inclusion of uixx objects properties in the manual (bug #46076) * doc/interpreter/genpropdoc.m: add uixx objects to the list of supported graphics objects * doc/interpreter/genpropdoc.m (get_doc): add uixx objects and their specific properties (currently empty documentation) * doc/interpreter/plot.txi("Interacting with Plots"): add a note and a reference about ui* family of functions. * doc/interpreter/plot.txi("Interacting with Plots"): for consistency, remove "uimenu" reference. All the other uixx are already in the gui section * doc/interpreter/plot.txi("graphics data structure"): add uixx objects * doc/interpreter/gui.txi("UI Elements"): add "uimenu" function reference * doc/module.mk: add rules to build uixx properties texi files. * graphics.in.h: make uixx "__object__" property (Octave internal) hidden so that it does not appear in the documentation.

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