Allow pausing/disabling static code analysis for single projects

When Creator analyzes multiple large projects, it often freezes or becomes so slow that productive working is hardly possible.

For example, I sometimes open qtbase as a secondary project to look up some internal of Qt. I have no intention to use the results of the static code analysis, I just want to be able so search for and read code. However, the analysis running in the background eats most of the resources that I'd much rather use on my primary project.