Warn the user when attempting to remove a node which has a knob which
is involved in an expression/link to another knob of the project.

Also don't allow selecting stuff in the roto panel for roto clones as
this messes-up the slave/masters relationships.
Fix deadlock in rotocontext
Use the proper rectangle for downscaling
Show the warning for deletion of a master knob only if the slave
belongs to a node
Restore correctly floating windows position
Spinbox increments now work according to the cursor position. Still
some work to do to remove extra 0 decimals
Pimpl spinbox
Handle decimals properly in Spinbox: don't show trailing 0's and create
decimals as necessary until we reach the maximum decimals set by the
plug-in for this parameter.
Fix bugs in the tab widget::closePane function
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop

Conflicts:
	Engine/EffectInstance.cpp
	Global/GitVersion.h