minor
Add 'm_' Prefix to member variable
Create a new unit test: TestLayer
Add virtual keyword to virtual function
Fix wrong include file
Fix bug of LayerImage::getMaxFramePosition(), remove duplicated method LayerImage::getMaxFrameIndex(), add responded unit test
Merge branch 'refactor/layer-manager' of https://github.com/chchwy/pencil2d into refactor/layer-manager