Add a setting for checking files upon saving

[VioletGiraffe]

Enabled by default, but users should be able to disable this feature.

[Altren]

Actually now it works next way: by default this setting is not set and whenever user save file for the first time he is asked:

"Do you want to start analysis any time file is saved? It will clear previous analysis results.
You can change this behavior in cppcheck settings."
"Yes"/"No"

So I see no reason to make this enabled by default, because user decide this once.

[VioletGiraffe]

I'm going to remove this question (#82). Besides, it should still be accessible after the initial decision.

[Altren]

It is accessible after initial decision in settings window.

[VioletGiraffe]

Oops, my bad, didn't notice it was added!