added isTrailingWhitespace method to DTCoreTextGlyphRun

This can be used to avoid drawing of background color, strikeout or underline for empty trailing white space glyph runs.
Implemented isTrailingWhitespace optimization

In two places: DTLinkButton + DTCoreTextLayoutFrame this now shortcuts for the trailing whitespace.

Also this fixes an underline alignment bug visible in the CurrentTest.html: if there are insets those need to be added to the origin of the run bounds.