Fix wxColour(NSColor) ctor and use it in wxOSX code

wxColour ctor from NSColor added in b478f24288
didn't work correctly as it didn't initialize wxColour::m_cgColour and so any
attempts to use the colour created by it resulted in an immediate crash (so
the code added in 16671f229a likely didn't work
neither).

It also assumed that the NSColor given to it was always in the RGBA colorspace
and crashed if it wasn't (so 56ebe6dfac fixed
compilation at the price of making the code crash at run-time). Now explicitly
request the RGBA colorspace and leave the colour uninitialized, which is
better than crashing, if it can't be obtained.

After making this ctor safe to use, there is no reason to reproduce its logic
in wxColourDialog and wxFontDialog, so just use it from there.

Also make the ctor explicit as it's a non-trivial operation which shouldn't be
performed implicitly and document that it doesn't take ownership of NSColor,
unlike the similar ctor from CGColorRef.

include/wx/osx/core/colour.h

@@ -58,7 +58,8 @@ public:
     wxColour& operator=(const RGBColor& col);
 #endif
 #if wxOSX_USE_COCOA
-    wxColour(WX_NSColor color);
+    // This ctor does not take ownership of the color.
+    explicit wxColour(WX_NSColor color);
     WX_NSColor OSXGetNSColor() const;
 #endif
     wxColour& operator=(CGColorRef col);