Various changes

New controls:
BasePage
PageUI
PickerBoxPageBase

Updated samples to take advantage of the new components.

ImageButton:
+ CornerRadius
* Padding had no effect (fixes #1)

ListPicker:
+ More samples
* More closely match the built-in experience

MenuItem:
* More closely match the built-in experience

PickerBox:
+ FullModeDisplayMemberPath
+ More samples
* No longer requires selection. No longer performs automatic selection.
* SelectedItems is now read-only and no longer a dependency property.
* More closely match the built-in experience (fixes #2)