Port some of the forked features

There are features in a number of forks that might make sense here as well. Just a pity that people don't offer them voluntarily in PRs 😭 😞 

e.g. https://github.com/ouoam/nodemcu-pyflasher 

- Disable Flash button when not select file
- Add support file drop
- Report error and info using Modal
- Show serial port description 
- Make flash label animate when flashing (or similar)