Rework parsing and provide unit test

Handling of parsing has slightly changed. The parser now uses simple
states to be capable of postponing triggered parses if there is already
a parsing in progress. Furthermore the parser now waits for the current
project to be completely scanned.

Change-Id: I6d4968d28194ba8d23d3a0ee6ab454d81a549e67
Reviewed-by: Andre Poenitz <andre.poenitz@theqtcompany.com>