Add text preview in lower-right corner

[JoeStrout]

Currently the lower-right corner of the screen shows a preview of the current glyph; this is leftover from the original fatbits program, and was intended mainly to let you scroll around a larger image.

For fontEdit, that sort of preview really isn't needed. Instead, what would be useful here is a preview of a string of text in the font, so you can ensure that the position of each glyph relative to the baseline and its neighbors is correct.

So, replace that preview area with a new one that has these features:

• Draws a multi-line string of text, with word break
• Draws a reference base line, i.e. a faint grey line at the Y position of each line of text, behind the text
• Automatically redraws as any glyph it contains is edited.

For now, we can use the fixed text "The quick brown fox jumped over the lazy dog." (In the future we'll add a button to let the user set this text, but that can be a separate issue.)