Debugger: Switch attach and setting of breakpoints

This avoids a race on Windows, where attaching is non-atomic.

Task-number: QTCREATORBUG-8663
Change-Id: I119cae7129882fabaa97fa3f236379e5efe62df9
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Update "getting and building" page.

With Qt5 private headers are (or can be) installed, so no need to build
Qt yourself.

Change-Id: I58e2eecabbb6c767a583d867167ad17eeb7b6f2a
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>