Bug fix in the settings panel construction. Adding groups to pages was
not supported, it is now.
Viewer: when connecting to a node with a shortcut, then set the viewer
as the selected node so the user has less clicks to make (and to match
what Nuke does).
Fix possible crash : only interact with backdrops with left click.
Fix previous commit
Roto: apply the same recycling of memory that we do in renderRoI. Now
when editing a mask Natron will not consume much memory
Stylesheet: QWidget add the colour attribute.
Nodegraph: check for sequential also when connecting the output
Prevent mis-usage of the natron background utility.

Also forbid viewer connection to sequential effects which are writers
only. Otherwise we would not be able to use ParticleIllusion from
Genarts.
Fix potential crash in RotoContext
Remove auto-saves when loading a project so that Natron will not try to
load autosaves when loading a specific project
Natron: remove the check for sequential plug-ins. We moved the check in
the WriteFFmpeg node
Handle premultiplication flag.
Bug fix in ViewerInstance::getFrameRange
ViewerCache: also apply the same recycling to memory that we do for the
Nodecache
For Parametric curves allow movements in both X and Y direction at the
same time.
OfxStringInstance: fixed a bug in the thread storage usage
Backdrop: adjust the size of the text automatically when resizing .
Add support for  kOfxParamStringEffectInstanceLabel
Fix previews on windows

Fix previews on windows: instead of setting black transparent bands
outside of the image set black opaque instead as the alpha is not really
supported for the QGraphicsPixmapItem on windows.
Fix previews for horizontal bands too