Replace the KritaDevs build type with a cmake option

Qt5's cmake scripts check the build type, and for anything other
than Debug append -DQT_NO_DEBUG, which disables Q_ASSERT. We
want to build with optimizations (-O3 -g) and without QT_NO_DEBUG
when developing Krita, so we catch the asserts and still have
a realistic performance.

CCMAIL:kimageshop@kde.org