Adds a variable "shouldAddFirstLineLeading" to DTAttributedTextContentView (and so to
DTAttributedTextView). I found that DTCoreTextLayoutFrame's _noLeadingOnFirstLine was
always no because layout frame was always being created anew and _buildLinesWithTypesetter
was always executed before it was set. This change causes _noLeadingOnFirstLine to take
the intended value immediately on layoutFrame's creation and so leading is applied or
not according to preference.
Merge pull request #378 from aeberbach/master

My fix for layout frame always adding leading to the first line