* changed implementation of executeProcess
* unified all different implementations of executeProcess
* separated arg list from command for all calls of executeProcess (this also required to remove quotes that were added to path args because otherwise the Python indexer would mistake absolute paths for relative paths.)
* update expected output for custom command tests
* Added new Source Group type SourceGroupCustomCommand to UI and ProjectSettings
* The Source Group executes a specified command on every source file
* The variables $SOURCE_PATH, $PROJECT_PATH, $DB_PATH, $STORAGE_VERSION can be passed
* The commands are executed after all other source groups finished indexing
* The command is executed with working directory set to project directory
* Errors during command execution are shown as status update and in the Status View
* SourceGroupCustomCommand cannot be partially cleared, when one file needs clearing the project must be fully re-indexed
* moved SystemHeaderSearchPaths and FramewordSeearchPaths of indexer cxx indexer commands to CompilerFlags section
* added SourceGroupTestSuite to test indexer command generation of all source groups
* removed "compile_commands.json" from gitignore, because a resource file for the tests has this name
