Add selector for `compile_commands.json`

[Discookie]

Closes #15.

Currently the database setup dialog has 3 options:

• Run CodeChecker log creates a terminal, with a pre-defined CodeChecker log command already written there.
• Locate shows a file selector to select the database's path, eg. for CMake with a /build folder
• Don't show again never shows the window again on folder open.

The dialog shows when compile_commands.json is not found. There's also a button in the Overview in an empty folder to select its path.

Currently there's no separation between missing the metadata.json and missing the compile_commands.json in the sidepanel. I'd like to add a separate sidebar panel option for it before merging.

[Discookie]

Added the sidepanel logic, should be ready now.

[Discookie]

Added watchers for all possible database paths, and the sidepanel should automatically refresh when at least one of them is found. Fixed most of the other issues as well.