Support full-screen cards

[nalbion]

The main purpose of this PR is to allow cards to be full-screen without the odd behaviour mentioned in #55.

In order to achieve this I've had to make a number of changes:

• Transform is used to translate and scale the card instead of Align and SizedBox
• Switching from SizedBox means that text on the card is resized instead of moved to different lines when the card size animates.
• Cards behind the front card are scaled in perspective, so you can have unlimited cards in your stack without cards at the back suddenly becoming larger
• Instead of specifying a minimum height/width, you can specify a scaling factor.
• card[0] is the front card - it removes a lot of complexity from the code
• the rotation angle depends on whether you started dragging the top or bottom half of the card. The card stays pinned under your finger and the far end drags behind.

[xxJeevesxx]

Do we know if this will get reviewed and closed?

[maheshj01]

cc: @ShaunRain