Add Gtk4 example using Plots package

examples/gtk4_plots_ex.jl

@@ -0,0 +1,35 @@
+using Gtk4
+using Plots
+using Printf
+
+c = GtkCanvas()
+
+function _plot(ctx, w, h)
+    ENV["GKS_WSTYPE"] = "142"
+    ENV["GKSconid"] = @sprintf("%lu", UInt64(ctx.ptr))
+    gr(show=true)
+    plot(randn(10, 3), size=(w, h))
+end
+
+@guarded draw(c) do widget
+    ctx = getgc(c)
+    w = width(c)
+    h = height(c)
+    @show w, h
+    rectangle(ctx, 0, 0, w, h)
+    set_source_rgb(ctx, 1, 1, 1)
+    fill(ctx)
+    _plot(ctx, w, h)
+end
+
+win = GtkWindow("Gtk4 example", 600, 450)
+win[] = c
+
+e = GtkEventControllerMotion(c)
+
+function on_motion(controller, x, y)
+    win.title = @sprintf("(%g, %g)", x, y)
+    reveal(c) # triggers a redraw
+end
+
+signal_connect(on_motion, e, "motion")