Plugin-loader handles .la files

Instead of passing .libs/plugin.so to plugin-loader, you can now
pass plugin.la and the correct file will be loaded.
Do not internationalize plugin-loader

There are no user-facing strings, so this is not necessary.
Update POTFILES.in so that 'make check' passes

Intltool has some intelligent checks for source files that look like
they contain translatable strings.
Add Glk unit test framework

This is the beginning of a unit test framework for Glk. Currently it has
only one test. But the potential is infinite! It could be used to test
how well other Glk implementations conform too.
Print more informative assert message

ASSERT_EQUAL() is for int-like types that will print with %d.
Unit test the datetime functions

Starting with these because of the recent change of the underlying
system.
Change I/O signals to pass a unique window ID

Change the 'char-input', 'line-input', and 'text-buffer-output'
signals to pass a unique window ID string as well as the
window rock value. If a client program wants to keep track of
input and output in all windows, then the window rock will
not do by itself - all open windows might have rock 0, for
example. The string ID is guaranteed to be unique for each
window, though its contents are implementation-defined.
Keep track of input and output in multiple windows in the player

In the ChimaraIF player widget, keep track of input and
output in all text buffer windows, not just one.