fix GTK_MAX_COMPOSE_LEN error message and increase value

Fixes #3521 (closed)
Helps with #2319 (closed)

#2319 (closed) points out that GTK_MAX_COMPOSE_LEN should probably be the max length, whereas as implemented it is the max length plus 1. However, grepping through the code revealed that some of the loops do i < GTK_MAX_COMPOSE_LEN+1 and others do i < GTK_MAX_COMPOSE_LEN, and I have no idea if there is any reason they should be different or if people just got confused about the array length. So it seems safer to make sure that there are no sequences of length GTK_MAX_COMPOSE_LEN.

I also increased the limit to support the examples from #2319 (closed) and #3521 (closed).