With QT_RESTRICTED_CAST_FROM_ASCII making GdbMi etc operate on
QString is feasible again. Take this as opportunity to move
debugger encoding handling closer to a 'conversion on input and
output if needed, storage in QString only' scheme.

Change-Id: I2f10c9fa8a6c62c44f4e6682efe3769e9fba30f7
Reviewed-by: Christian Stenger <christian.stenger@qt.io>