fix a bug in visual cursor movement

fix a bug in visual cursor movement

The setting of boundary need to take the paragraph's direction (LTR/RTL) into
account.

This bug was discovered by Vincent, while trying to solve bug #5061.

While this fixes the original manifestation of bug #5061, it doesn't really
solve the underlying issue, which is that on a boundary, after typing
Ctrl-Enter, the cursor is painted on the same line as the newline symbol
rather than on the new line. See bugzilla for further discussion of this.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@27225 a592a061-630c-0410-9148-cb99ea01b6c8