Save .png only
AtlasFrame.glyphSheet modifications

Details:
-made private
-made into an Option as it is not initially loaded
-added PNG file filter
Remove unnecessary constants from AtlasFrame

Details:
-added GlyphSheet.getNumGlyphs
-kept SPRITE_SIZE and made it public
Correct chopped descent

Also:
-fixed bug where typeface wasn't used in getMaxPtSize
Added GlyphMetrics
Save GlyphMetrics

Also:
-moved boundingRects from GlyphSheet to GlyphMetrics
Save GlyphMetrics in more readable format
Added Open/load functionality

Also:
-fixed bug in GlyphMetrics.fromXML
-added getMetricsFile
-clear currentFile and title on New...