Fix showing diagnostics in the Problems pane

[csordasmarton]

Closes #49

• Show reports as separate diagnostic items and show bug steps as diagnostic
  related information.
• Show different severities based on the CodeChecker severity level.
• Refactoring the code.

Screenshot:

[Discookie]

There's a couple things broken by this change:

• The reproduction steps are now invisible in the editor, other than the CodeLens header. Makes it much harder to see eg. when there's multiple statements on the same line, or when only a certain parameter is of interest. (CodeChecker web highlights the steps in yellow.)
• Reproduction steps are always shown in the Problems tab, so when the user clicks on a non-active repr. step, and the step is in a different file, the entire bug disappears.
• There's now a considerable amount of duplicate information between the Reports tab and the Problems list, but with the Problems list having worse functionality.
• Since keyboard navigation is based on the repr. steps' ranges, it becomes very unintuitive as well since the user won't be able to find the range where the navigation works.
• Small nitpick is that the report's description always contains a link to itself as the last step.

My recommendation would be to have the steps displayed only under the active report, but still have the individual steps be displayed as Information ranges.

Another thing is using the other diagnostic severities for different kinds of reports, so that Low severity can become a warning instead of an error, and so on.

[csordasmarton]

As we discussed this on a meeting we will remove bug steps from the Problems pane and I created an issue (#88) to highlight bug step ranges in the editor different way if it's supported.

[Discookie]

LGTM!