Gui: fix the QLineEdit selection color, +tidy up

once again, this was not a Qt bug
Gui: add all files to .pro
Node: be safe when in release mode (I met this assert at least once)
ViewerGL: factorize + use updated GuiMacros
EffectInstance: work around inputIdentity==-2 bug

and remove lengthy and useless « else »
Image: add assert
SpinBox: complete rewrite of position-based increment

previous code has many bugs, especially when approaching or crossing
zero.
This one may have bugs, but at least it has comments!
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop

Conflicts:
	Gui/GuiMacros.h