Avoid invalid Python escape warnings

[Mirochill]

Summary

• use raw strings for escaped Robot Framework variable names in RemoteSwingLibrary.py
• escape backslashes before writing SwingLibrary documentation into generated Python string literals

Fixes #125.

Validation

• Not run locally
• rg -n "set_global_variable" src/main/python/RemoteSwingLibrary.py
• rg -n "docs = docs.replace" src/main/java/org/robotframework/remoteswinglibrary/build/SwingLibraryKeywords.java
• git diff --check
• git diff --cached --check
• git diff --check HEAD~1..HEAD