Made bank-select configurable.

Made the bank-select table configurable instead of hard-coded.  The
configuration file should contain a spec. that looks like this example:

bank_select_start
GM;0;0
Pre1;63;0
Pre2;63;1
bank_select_end

bank_select_start and bank_select_end, of course, start the bank-select
table definition.  In between, each line specifies 3 values:  The name of
the bank represented by that spec., the most significant bit (MSB), and
the least significant bit (LSB), with the fields separated by a semi-colon
(or, optionally, a comma or a period).  Added to config1 a bank-select
configuration for my Motif XS keyboard (which was previously hard-coded in
BankSelect_MIDI_Event.pm).  Other users can now configure a bank-select
spec. for their own keyboard models, using the new spec in config1 as an
example.
Cleanup after configurable bank-select implementation

Removed old_... (obsolete) subroutines, etc.  Copied the Motif XS
bank-select table spec. from config1 to config[23]; added some user
documentation, in the form of comments, in the config files explaining
the format of the bank-select table specification.
Updated README file with respect to bank-select.

Removed the "Current Limitations" section from the README file - the
only thing mentioned was lack of configurable bank-select
specifications, which has now been added.