imwayland: allow some range of serial numbers

Some input method like ibus-m17n will send two consecutive events together with two done events. Then the first preedit_string and done request will increase the serial number locally, and send the commit request to mutter. But the second commit_string and done event will be ignored, because the serial number is changed locally.

As the the data of text input protocol is sent over network, we can use the technique like the network sliding window protocol, the commit request will increase the window, the done event will shrink the window. Thus we allow some range of serial numbers in the text input protocol.

Fixes #3641 (closed)