Commands FSSplitLeft (..Right.., ..Above.. and ..Below..) will
temporarily unset 'splitright' and 'splitbelow' options and revert
them back after splitting. This will fix behaviour of these commands
when the options are set.
Option 'switchbuf' is checked in function FSwitch() to ensure that
already open buffers will be jumped to rather than opening a new
window when its value is 'useopen' or 'usetab'. Jumping to an already
existing buffer is achieved by using command sbuffer which knows how
to do when switchbuf is 'useopen' or 'usetab' (similar solution can
be found in a.vim).