Changed Windows sysData.setText() to panic on error. WM_SETTEXT's documentation indicates the main error is memory exhausted, which means we should panic anyway (unless the Go runtime already has). Any other error will be flagged as unexpected; since we're using SetWindowText() we can't differentiate between out of memory and other errors in the way WM_SETTEXT documents anyway.
Removed error returns from sysData.setText() now that we no longer care.
Removed error returns from [all controls].SetText() and Window.SetTitle().
Fixed compiler errors in the previous few commits.
More TODOs.