Don't ask for a file on 'insert source' if a path is selected
Insert snippets of text

Snippets can be defined in 'Options -> Snippets...'. Each one is associated
to a tag.  To insert a snippet, type its tag and press 'Edit -> Insert ->
Insert snippet' (F7).  If no tag matches at the cursor position, a list is
shown to choose one.
Add HTML export

Editra was a helpful resource for implementing this; many thanks to
Cody Precord
Add three new text styles

The styles are 'unknown function', 'parameter' and 'assignment'.  This
also fixes variables and parameters being highlighted as functions when
there's a function with the same name, at the cost of functions without
parentheses not being recognized as such.

See issue #14
Add a reset defaults button to the 'fonts and colors' dialog
Allow customizing the default basename pattern used for saving images

When saving an image, the output basename is checked for the script's
title and frame number.  The pattern is saved for the next time an
image is saved from the same tab.  This other pattern can be set in
'program settings' to be used when switching tabs.