Save as Image dialog fixes and improvements

Here's two small improvements to the save as image dialog. The first replaces the current, limited custom dialog with the standard file save as dialog under linux. I'm not sure why previously only Windows and OSX used the standard file dialogs - but in my testing the standard dialog works fine and has much improved functionality. The second commit adds the ability to specify a default filename for the dialog - this will be very useful for certain types of composer outputs.