* Added a tool to adjust image brightness and contrast. Useful for dark images.

* Refactor brightness and contrast adjuster

* Fix syntax error in Python2
Co-authored-by: NKentaro Wada <www.kentaro.wada@gmail.com>

* Fix for flake8

* feature(app): open files by drag and drop

* refactor(app): ignore events without right ext.

Ignore dragged items that don't have allowed extension.
Make extension global variable to avoid repetition.
Fix line length.

* fix(app): fix flake8 errors

* Refactoring
Co-authored-by: NKentaro Wada <www.kentaro.wada@gmail.com>

When editing a label with an group ID and u delete it, labelme crashed:
Fixed error "TypeError: setData() takes no keyword arguments"
Deleting the keywords fixed this error

```
labelme data_annotated --labels labels.txt --config
  '{shape_color: manual, label_colors: {person: [0, 0, 255]}, default_shape_color: [255,0,0]}'
```