Log Message:
Update exmh package from 2.5 to 2.6.3.

Requested by Steve Bellovin in PR 21507

Changes from previous version:

2.6.3	fixes a UI pause problem introduced in 2.6
2.6.2	bug fixes
2.6.1	generalised sequence handling
	- replace the "unseen" window with a sequences window, and add a
	Sequence menu.
	- display number of unseen messages in folder cache
2.6	Add a Sequences menu, and internal overhaul of sequence handling