Added scripting functions SelectGlyphsSplines, SelectGlyphsReferences

Added scripting functions SelectGlyphsSplines() and
SelectGlyphsReferences(), respectively for selecting glyphs with only
splines and only references.  (SelectGlyphsBoth was already present.)
These functions work similarly to the menu commands in the UI; they
take no arguments.
Added ChangeWeight native scripting function.

Added ChangeWeight() function for boldening and unboldening.  It takes
one argument which is the weight change in em units.  Positive values
make the selected glyphs bolder; negative values make them less bold.
Added "Class" parameter to GlyphInfo native scripting function.

GlyphInfo() may now be passed the string "Class" as its first
argument, and it will then return a string corresponding to the glyph
class of the selection, which will be one of "automatic", "none",
base", "ligature", "mark", "component", or "unknown".
Documented SelectGlyphsReferences and SelectGlyphsSplines.

Also added index entry for SelectGlyphsBoth which was missing.
Documented the ChangeWeight() native scripting function
Documented the Class parameter to GlyphInfo()
Merge pull request #1055 from blipvert/fixes/21

New scripting functions: SelectGlyphsSplines, SelectGlyphsReferences, ChangeWeight, and "Class" tag to GlyphInfo()