Improve searching

Searching in a huge mail box is buggy and takes a long time. The reason is that after each typed character a new search starts.

• Add a timer such that a search query is send only after a couple of milisecs with no input. there's now a configurable delay in InboxViewController before the search is executed
• Can you improve body search queries ? see update
• possibly refactor search in InboxViewController

UPDATE 26.02: the search now looks for the separate terms in a scrambled order as well. A strict order can be enforced if the search is wrapped in "". So car box looks for either of the two words in whatever order, "car box" matches the whole string