Tidy up NS color handling

Tidy up NS color handling

* src/nsimage.m (COLORSPACE_NAME): New macro to find the current
colorspace.
([EmacsImage initFromXBM:width:height:fg:bg:reverseBytes:]):
([EmacsImage initForXPMWithDepth:width:height:]): Use the current
colorspace.
* src/nsterm.h (NSAppKitVersionNumber10_7):
(NSAppKitVersionNumber10_10): Define for macOS version checks.
* src/nsterm.m ([NSColor colorForEmacsRed:green:blue:alpha:]): Tidy up
the version checking.
([NSColor colorUsingDefaultColorSpace]): Tidy the version checking and
use [NSColor colorUsingColorSpace:] with GNUstep.