Updated PDF Tab, Started Media Tab

The PDF tab is now much more functional. It's actually pretty nice to
view a PDF with. I added some buttons to zoom in and out as well as
change pages.

The Media tab has a skeleton to it. The PDF Tab was moved to a new class
instead of burying itself in the MainWindow class. When I start fleshing
out the Media tab, I'll be making sure it's in its own class.