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.
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.