Adds a Flag grob to LilyPond.

Adds a Flag grob to LilyPond.

This results in more flexibility with the control of flags and their
attachment to stems.

It also results in more accurate skylines for horizontal spacing, which
tightens up certain loose spacing situations, especially ones involving
grace notes that are close in pitch to the notes that follow them.

Note that this patch prolongs a problem with the calculations of flag
widths, which are artificially set at a half stem width skinnier than
they are actually.  The same problem existed in the old Stem::width
function by using Stem::flag instead of Stem::get_translated_flag
to determine the width of the stem grob.  This can be fixed in one
of several ways, but my recommendation would be to fix it in the
Feta font by adjusting the set_char_box for flags.