Fix Alt+[0-9] buffer access order

WeeChat sends them in no particular order, we need to sort the buffers by
their WeeChat number. To avoid copying the potentially very large buffer
objects around needlessly, extract the relevant keys and sort, then access.

This is based on https://github.com/ailin-nemui/glowing-bear/commit/ad50220bfdfcf029fc47093ce5cfea04bf90fe79
Merge pull request #431 from glowing-bear/fix-buffer-ordering

Fix Alt+[0-9] buffer access order