Create class to resize images

The new file dialog in the .glade file should also be used for this resize class. Perhaps call it resizedialog.cc and resizedialog.h.

To elaborate this will essentially be the same code you see in newdialog.cc and newdialog.h. The only difference being the code for the ok button (currently newDialog::add_page) which will instead take the values from the imageWidth and imageHeight spinbuttons and execute a method in canvas.cc to resize the image to those dimensions. Inside the resizedialog.cc and resizedialog.h just replace everywhere you see "newDialog" with resizeDialog.