Add basic filtering to the UI: level, kind, important

[yairlenga]

I'm working to finalize integration of Coverity analysis into vscode, via the Sarif viewer. This is major improvement to developers' productivity vs using the Coverity connect. However, there are few things that exists in connect - which will make life much easier:

• Simple filter to specify "level" (errors, warnings or notes). Can be implemented as minimal level to show (e.g., warnings will show warnings + errors), or just a 4 way filter: all/errors/warnings/notes).

• For showing issues by location - files are shown by number of messages. Will be nice to allow showing the files by name (as a tree). Something like

src     (500)
    lib (300)
        file1.c (150)
        file2.c (200)
    prog/
        main.c (12)
        utils.c (15)

Will save lot of navigation time.