Fix updates of the color selector when color space of the image or node is changed

IMPORTANT:
This commit introduces a KisSignalsBlocker class which does scope-defined
blocking of signals of a QObject-based. Please use it instead of direct
calls to object->blockSignals(true) when writing new code. The latter way
is purely unsafe (from 'return' and exceptions point of view).

CCMAIL:kimageshop@kde.org

The rest of the patch fixes updates of the color selectors in the following
cases:

1) Image color space is changed
2) Current node color space is changed
3) Image projection color space is changed
4-6) Undo of the cases 1-3 [PLEASE TEST IT]