logic: Added SourceGroupCustomCommand to use with SourcetrailDB binaries

* 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

src/lib/settings/SourceGroupSettingsCppEmpty.cpp

@@ -51,14 +51,9 @@ bool SourceGroupSettingsCppEmpty::equals(std::shared_ptr<SourceGroupSettings> ot
 	);
 }
 
-const ProjectSettings* SourceGroupSettingsCppEmpty::getProjectSettings() const
-{
-	return m_projectSettings;
-}
-
 std::vector<std::wstring> SourceGroupSettingsCppEmpty::getDefaultSourceExtensions() const
 {
-	return { 
+	return {
 		L".cpp",
 		L".cxx",
 		L".cc"