Remove unnecessary file existence check in GMenu2X::readTmp()

A file existence check is already performed, atomically with respect to
the filesystem, by ifstream's constructor. The result of this check is
available using ifstream::is_open().
Remove unnecessary file existence check in GMenu2X::writeSkinConfig

mkdir is able to return an error code in errno to indicate that the
directory requested to be created already exists (EEXIST).
Remove unnecessary file existence check in GMenu2X::showSettings()

Browsing the user's translations directory will simply add no new files
to the list of translations available if the directory doesn't exist.
GMenu2X::showSettings() doesn't need to care about that.

Because translations are files, not directories, also don't return
directories in the result. The code that makes the translations list
calls FileLister::getFiles and ignores directories anyway.
Remove unnecessary file existence check in WallpaperDialog::exec()

Browsing the user's wallpaper directory will simply add no new files
to the list of wallpapers available if the directory doesn't exist.
WallpaperDialog::exec doesn't need to care about that.

Because wallpapers are files, not directories, also don't return
directories in the result. The code that makes the wallpapers list
calls FileLister::getFiles and ignores directories anyway.
Make FileLister::browse not log a non-existing directory as an error

To avoid unnecessary existence checks, we consider browsing a
non-existing directory to be a normal situation.