Started development on "exclude from group" feature #254

[x] Data sources can be temporarily excluded from their grouped data source
[x] Prominently show that a data source is excluded in the data source tree
[x] Persist "is excluded" for data sources that are part of a merged data source
[ ] Excluding a data source from its group causes the total number of errors, etc... to change - this is contrary to how filters behave which in fact do not influence the counters at the top
[x] Added tests for new EmptyLogFile class
[x] Added tests for excluding / including a log file from its group
[x] Added tests to verify that excluding / including works for both merged- as well as folder data sources
[ ] Added documentation to explain this new feature

Fixes #254

Kittyfisto / Tailviewer