Mono.Cairo: Add Context.InClip method

The cairo_in_clip function was added in cairo 1.10.
Mono.Cairo: Add Context.HasCurrentPoint property

The cairo_has_current_point function was added in cairo 1.6.
Mono.Cairo: Add Context.SetSourceColor(Color) convenience method

The Color property was obsoleted, but the suggested replacement
SetSourceRGBA has 4 double parameters, which can be unconvenient.

Add a SetSourceColor that takes a Cairo.Color parameter, make the Color
property use it, and update the message on the Obsolete attribute.
Merge pull request #1071 from bl8/cairo-additions

Cairo API additions in Context class