_gtkButtonLabelSet set the label of a Buttons
object.
For the sub classes ButtonColor and ButtonFont, this set the title of the
resulted dialog box, not the button itself.
Prototype : fun [ObjGtkButton S I] ObjGtkButton
ObjGtkButton : a Buttons object.
S : a string (the label).
I : has a mnemonic (1) or not (0).
If characters in the string are preceded by an underscore, they are
underlined. The first underlined character represents a keyboard
accelerator called a mnemonic. If the user presses Alt and this key
then the button is activated (clicked).
Return ObjGtkButton : the same object or nil if an error occurs.
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