TweetListEntry: Make seen an explicit property

Will be needed in later commits
TweetUtils: Remove g_message call
update POTFILES.skip
Add highlighting test case
Merge branch 'master' of https://github.com/baedert/corebird

Conflicts:
	src/util/TweetUtils.vala
	tests/highlighting.vala
TweetUtils: Treat underscores in mentions right

Fixes #237
tests/highlighting: Fix test case
pot update