Add slider (GtkScale) example

docs/make.jl

@@ -21,6 +21,7 @@ makedocs(
                      "manual/keyevents.md",
                      "manual/canvas.md",
                      "manual/customWidgets.md",
+                     "manual/slider.md",
                      "manual/async.md",
                      "manual/nonreplusage.md",
                      "manual/packages.md"

docs/src/manual/slider.md

@@ -0,0 +1,36 @@
+# Slider widgets (aka GtkScale) and dynamic adjustments
+
+The following example
+creates two sliders using `GtkScale`.
+The callback function for the 1st slider
+dynamically changes the value of the 2nd slider
+to force it to match,
+and also dynamically changes the range of the 1st slider
+if the value reaches 10.
+
+This example illustrates
+that one can use `GAccessor.value`
+both to _access_ a widget value and to _alter_ that value.
+
+```julia
+using Gtk: GtkGrid, GtkScale, GtkWindow, GAccessor
+using Gtk: signal_connect, set_gtk_property!, showall
+
+win = GtkWindow("Sliders", 500, 200)
+slider1 = GtkScale(false, 0:10)
+slider2 = GtkScale(false, 0:30)
+signal_connect(slider1, "value-changed") do widget, others...
+    value = GAccessor.value(slider1)
+    GAccessor.value(slider2, value) # dynamic value adjustment
+    println("slider value is $value")
+    if value == 10
+        GAccessor.range(slider1, 1, 20) # dynamic range adjustment
+    end
+end
+g = GtkGrid()
+g[1,1] = slider1
+g[1,2] = slider2
+set_gtk_property!(g, :column_homogeneous, true)
+push!(win, g)
+showall(win)
+```