change tracking:

change tracking:

	* changes.[Ch]: introduce isSimilarTo(...);
	restore original operator==;
	When merging two adjacent changes, the changetime is not considered,
	only the equality of the change type and author is checked (in method
	isSimilarTo(...)). If two changes are in fact merged (in method merge()),
	the later change time is preserved.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15608 a592a061-630c-0410-9148-cb99ea01b6c8