Fix Copy From Source Text to work on OS X.

Doing this action no longer updated content in the list control.
Broken by a007e1d528e3.
Use small font for explanations in preferences.

Use small variant of text on OS X and gray text on Windows for
explanatory notes in preferences.  This was already done on the TM page,
do it elsewhere as well.
OS X: Fix reporting of missing spellchecking dict.