Performance of frame changing has been increased due to UI containers…

Performance of frame changing has been increased due to UI containers had detached from DOM before their items were created

Bug has been fixed: Failed to execute 'inverse' on 'SVGMatrix': The m…

Move function from closure to class method

- changed Content-type for save_job request to application/json, object…
- Adopted public PR - PASCAL VOC converter
- Added convert_to_mask.py script
- Fixed player continue plaing at the end of video, lock for editable object broke the client
- Same colors for shapes and menus, ability to change color for label or group
- Undo redo
- Added license header for all files
- Added .gitattributes file (critical for bash scripts)
- Fixed "Don't delete tasks if a user is deleted"
- More strict check for 'checkbox' and 'number' values
- Added convert_to_coco.py script
- Job statistic were extended. Blowradius was removed
- More strict labels verification
- Drag polygons by requirement, norm stroke opacity, easy box dragging
- Drawing with mouse outside the image area
- Fixed 7z support
- Boundig box size in drawer, switcheable grid
- Split tracks feature
- Fix flip parsing
- Update color set & color by label feature
- Add point for polygons feature
- Added context menu
- Polyshapes

Feature/pascal voc converter

unit testing, warns when using interpolation xml's or images with annotations different than bounding boxes

enable the import of picture with alpha channel

 - jpg doesnt support alpha channels
 - when importing pictures with alpha channel error shows up
 -- "Could not create the task. OSError: cannot write mode RGBA as JPEG"
 - fix error by converting "RGBA" to "RGB" before saving image

* Fixed 7z support. (#235)
* Added 'Development Environment' into CONTRIBUTING.md
* More strict verification for labels with attributes.
* More strict check for values of checkbox/number type of attributes.
* Added .vscode/launch.json

Changes:
- Fixed https://github.com/opencv/cvat/issues/14 (docker-compose down command as written in the readme does not remove volumes)
- Fixed https://github.com/opencv/cvat/issues/15 (All checkboxes in temporary attributes are checked when reopening job after saving the job)
- Fixed https://github.com/opencv/cvat/issues/18 (Extend contributing.md)
- Fixed https://github.com/opencv/cvat/issues/19 (Using the same attribute for label twice -> Stuck)

    * video from share hotfix & dump hotfix
    * Added contributors
    * Added screenshot, documentation, screencasts into README.md
    * Fixed GitHub documentation