Set font in BufferView::setCursor (#9500)

Set font in BufferView::setCursor (#9500)

This solves a few bugs related to the font not being set correctly.
For example, when putting a selection somewhere with
putSelectionAt(), the font was not reset so that before this commit
if the cursor was in an ERT, strange things could happen.
putSelectionAt() is notably used when highlighting the location
corresponding with a LaTeX error (GuiErrorList), when using find,
and when using the spellcheck. I could reproduce the bug using all
three of these.

Bug #9500 is an example of the type of bugs that this commit fixes.