Metricsview: allow chars to be created on demand

You can now type a char in and fontforge will create that char in the
font if it doesn't already exist. Handy for setting the glyph width
regardless of if there is already a glyph in the font or not.
a warning sign
Merge pull request #1925 from monkeyiq/2014nov/metricsview-create-char-on-demand

Metricsview: allow chars to be created on demand