Debugger: Rework display type selection

The previous index based way was getting too brittle, use enums instead.
Also add a switch between exponential and flat display for floating
point types.

Task-number: QTCREATORBUG-12050
Change-Id: I86addbac5a80e8b79b176c6107b251b466503fe7
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>