Remove deprecated functions in test programs

Some of the test programs were using things like GtkHPaned and
GtkHButtonBox.
Port date/time functions to GDateTime

These are supposedly portable to more platforms than POSIX, effectively
putting the burden of checking whether a platform supports it onto GLib,
instead of having Chimara shoulder it.
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.