Replace GtkObject with GObject

In gtk-2.0 both are the same in gtk-3.0 GtkObject has been removed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
plugins/AD7303: Use gtk_range_{set,get}_value

Use gtk_range_{set,get}_value instead of directly modifying the adjustment.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Use gtk_widget_set_tooltip_text() instead of the old GTKTooltips API

Using the GTKTooltips API is deprecated, use gtk_widget_set_tooltip_text()
instead. This also makes the code a bit shorter.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>