hodgepodge for test_shell.cc

This is my work in progress to get TestShell working.
It doesn't work yet, and it also includes a bunch of other patches that I've already uploaded as other reviews, so I expect it to get smaller as they are committed.

The plan is to have a general TestShell with a pointer to a platform-specific TestShellImpl.  The test_shell_gtk.h TestShellImpl shows what an empty one might look like.  The test_shell_win.h one obviously won't work.

Places marked XXX I intend to fix before sending this out; places marked TODO(port) or NOTIMPLEMENTED I intend to put off until after committing.