Need better documentation

[leofang]

This includes at least

1. docstrings for GUI functions
   • thoughts: most GUI methods are considered "private" in the sense that we are free to make changes to them as long as the effects are obvious to users. Users click buttons, not call the methods, after all.
2. document or note for how new functions are added
   • pyqt style file and auto-gen script
   • main GUI class
   • interface class
   • backend
3. cleanup to reduce future maintenance burden

This needs to be resolved soon so that when I am unavailable or stepping down from the maintainer role, Max Rakitin (@mrakitin) or LonglonWu can take over.

[leofang]

4. Need text descriptions for GUI elements: When the cursor hovers over buttons or input fields, currently only a few items pop up a text instruction. We should make it more self-explained and so user-friendly.