Initial multi-root support

[Discookie]

Requires #123. Reference: #113, Ericsson/CodeChecker#3708

Adds support for CodeChecker 6.22's automatic compilation database detection. If no comp.db. is specified, and there is none in CodeChecker's output folder, the extension will use CodeChecker's autodetection (for 6.22 and above). For 6.21, and the mentioned cases, the behavior remains the same.

Remaining tasks:

• Design and implement commands for building multi-root projects. Currently you can only build either a single file, or the outermost folder via the default commands.
• Support for anayzing 2 or more files without an explicit comp.db. From my reading, CodeChecker analyze can only accept a single file or folder as input, so for multiple files we'd need to autodetect common folders, or run the analysis for each file separately.
• Handle failure of CodeChecker analyze if it does not detect a compilation database. Currently it returns code 0, so we'd need to read its output to detect the failure.

[Discookie]

Rebased, should be ready for review now. Requires #125.