Move calls to selsnap into selnormalize

This simplifies getbuttoninfo() and bpress(), and fixes a bug which made word
snapping behave incorrectly when a delimiter was at the beginning or end of
line.

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>
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Gitea Version: 1.28.0+dev-505-g31b4d79a84