[lmi] Input-sequence editor integration

[Greg Chicares]

I figured I'd start the new month with a new thread, because we outgrew
the old name "sequence input editor -- how to get accepted keywords", and
because here I'll approach it bottom-up instead of top-down. I'm breaking
the work into lots of little revisions, and I'll post random notes in the
hope that any mistake in my thinking will be spotted easily.

20100701T1731Z (revision 5018)

In 'Makefile.am', I think 'test_input_SOURCES' should contain
'datum_sequence.cpp', but doesn't need 'input_sequence_entry.cpp'.

(planned for a subsequent revision)

If we removed datum_string::operator==(), this would result:

/lmi/src/lmi/any_member.hpp: In member function `bool holder<ClassType, 
ValueType>::equals(const placeholder&) const [with ClassType = Input, ValueType 
= datum_string Input::*]':
/lmi/src/lmi/input.cpp:627:   instantiated from here
/lmi/src/lmi/any_member.hpp:202: error: no match for 'operator==' in 
'*((&*(Input*)z->holder<Input, datum_string Input::*>::object_) + 
((int)z->holder<Input, datum_string Input::*>::held_)) ==
*((&*(Input*)((const holder<Input, datum_string Input::*>*)this)->holder<Input, 
datum_string Input::*>::object_) + ((int)((const holder<Input, datum_string 
Input::*>*)this)->holder<Input, datum_string
Input::*>::held_))'

...so I think it'll prove easier to implement the same operator for
class datum_sequence now.

I note one curiosity in passing: 'datum_boolean.?pp' seems to be unused.