[JENKINS-14481] Quickly identify only the new errors

- A new column added to the report details table. It can contain "new", "solved" or "unchanged" flags to distinguish state of comparison with the previous report (if any). The column is mainly defined for an ability of sorting.
- "New" issues are additionally highlighted with red background, "solved" issues are green and "unchanged" has white color as usual.
- Solved issues also don't have the links to the source code, because it isn't available (issue was solved and the code is no longer present).
- New CppcheckDiffState enum/flag added and stored as transient in CppcheckWorkspaceFile class. It is a dynamic parameter that should not be saved to any XML file.
- The real comparison is done in CppcheckDiffState.diffCurrentAndPrevious().
- If source file is not available, no line will be displayed in the report (zero was present before).
Merge pull request #6 from mixalturek/master

[JENKINS-14481] Quickly identify only the new errors