Added Luma Keyer image filter

* Added luma keyer image filter
- Added luma keying curves paint delegate
* Minor bug fixes on curves widget
* Fixed the tooltip texts wher pertinent to make them follow the same
format
* Added magnifying glass, interpolation and invert icons to the widgets
project resources
* Changed the form "EndPoint" to "Boundary" in the identifiers of the
cubic spline interpolator (e.g: from "EndPointConditions_Natural" to
"BoundaryConditions_Natural")