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

testing/project_setup/custom_command/data/add.py

@@ -0,0 +1,6 @@
+import sys
+# import time
+# time.sleep(1)
+
+with open("out.txt", "a") as myfile:
+    myfile.write(sys.argv[1] + ' ' + sys.argv[2] + ' ' + sys.argv[3] + ' ' + sys.argv[4] + '\n')