In Glade, I go to add a button, and try to add a label/image; however, the stock button and label fields are faded, and I cannot add anything to them. Why does it do this? How do get Glade to allow me to add content to the buttons?
You must add a Related Action by clicking the "..." button. Once you have added an action, disable Use Action Appearance. Then you can clear the related action and set the button to whatever you wish.
You can also continue using the action as sort of standardized actions in your program. For instance, most apps will have new buttons, so with actions you can create a simple new action, then just set toolbar buttons, buttons, whatever to that action and get all the related callbacks, icons and titles.