lib/gtk: fix duplication of "visible" properties

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: fix name collision of the "visible" properties

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples/calculator & lib/gtk: fix whitespaces

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge pull request #1 from xymus/calculator

Small fixes for the calculator example and gtk lib