Frontend: execute /exec output in the same thread

We can't use the TaskQueue thread via Execute() as Execute() of /exec itself
already blocks the queue as long as the /exec process does not terminate.
This prevented /exec -o tail -f foo to work.
Engine, Frontend-GNOME: removed UrlMessagePartModel.Protocol
Engine(-Tests): implemented SmartLinks with some basic patterns (closes: #498, #911, #923)
Engine-Tests: added benchmarks for smartlinks
Engine(-Test): removed MessageBuilder.AppendRichText()
Engine(-Tests): applied coding standards
Engine-Tests: added test-case for broken mailto link
Engine: refactored MessageBuilderSettings for cleaner API, fixed mailto: and Smuxi bug links
Engine-IRC: make use of smart links in IrcMessageBuilder
Frontend-GNOME: fixed dealing with mailto: links in Frontend.TryOpenChatLink()
Engine: added a bunch of common bug trackers, CVE, DSA and RFC as smart links
Engine-Tests: fixed test-case for broken mail links
Merge branch 'stable'