Missing a way to close plot windows

[ma-laforge]

Request derived from a question on Discourse (Iulian.Cioarca):

https://discourse.julialang.org/t/how-to-close-inspectdr-plot-window-or-update-it/25787

InspectDR should have a cleaner interface to close plot windows. Right now, we must use non-public functions:

using Plots
using InspectDR
inspectdr()
plot(1:100)
plotobj = gui()

InspectDR.window_close(plotobj.wnd)

[ma-laforge]

Can now close a plot window using:

close(plotobj)

Also added a way to clear plot data using one of:

• clear_data(p::Plot2D)
• clear_data(p::Multiplot)
• clear_data(p::GtkPlot; refresh_gui = true)