Revert "Change default working directory from ~/ to ".""

Revert "Change default working directory from ~/ to ".""

Setting the default working directory to '.' is an invasive change
on Mac and Windows, where the current working directory of LyX when
LyX is started from the GUI is not a reasonable choice for the
default directory in e.g. the "save as" file dialog or the "open"
dialog. A similar situation could happen on Linux distributions (I'm
not aware of any, e.g., POSIX standard that defines that the working
directory of a binary started from GUI should be the home
directory). Thus, it is not clear this default is good on any
platform.

If the user wishes to have the behavior of '.', the description in
the "paths" preferences pane provides the information for making the
change.

For more information, see the discussion at:

  https://www.mail-archive.com/search?l=mid&q=635298DE-5028-4BB1-8D8F-A634BA68C420%40gmx.net

This reverts commit 892593fbdc25b9f8b413dd842748e0b3a57c90b0.