Provide enough information for "prompt" to be toolable

Qmake got a builtin "prompt" functionality that can be used to query input from users. Please extend that command so that it can provide more information to Creator, so that it can ask the user in a GUI way.

A way to provide more context (e.g. "press 'c' for commercial, ..." as found in configure.pri) and a default value (so that less important questions can be skipped) are currently missing and make it impossible to have a nice UI for the "prompt" builtin.

Being able to provide a set of allowed values would be appreciated, too, so that we can have a better UI.