Fix dialog spacing and other MDDialog enhancements

Started this MR because I realized the padding was wrongly applied to the AnchorLayout instead of the inner GridLayout. Also fixed some annoying kv-language spacing issues and added a ScrollView to hold the content container. The latter required some small changes in kitchen_sink.py, but the upshot is that content which is large than the dialog size will now scroll properly. Oh, and that there are now two buttons in the Dialog screen rather than one.

Fairly simple changes so shouldn't be a problem to merge, but one thing to point out is that due to my own personal taste the ScrollView defaults to 'ScrollEffect' (i.e. no overscroll). Seems the most intuitive behavior to me in dialogs (where overscroll is at the very least uncommon).