Paragraph::isLetter(): Fix word selection in ERT. The check had nothing to do with...

Paragraph::isLetter(): Fix word selection in ERT. The check had nothing to do with spellchecking. This method is misnamed, it should perhaps be renamed to 'canBePartOfWord()'.

NB: I am not sure this escape character preference is something useful to keep. If some language has some special escape characters beside the single quote one we should support that out of the box and not asking the user to define it or them in the preference dialog.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@30210 a592a061-630c-0410-9148-cb99ea01b6c8