Add 'override' keyword to virtual functions
Add 'm_' prefix to member variables
Code cleanup
Remove out-dated info
Misc
New signal/slot
Add 'widgets' qt package
Remove unused variables
Cleanup
Cleanup
Layer refactoring... not finish yet.

Cannot compile now.
Step 1, compile ok.
..
Step 2. re-implement LayerImage
Try to remove LayerImage, use Layer instead.
1 more unit test
Merge branch 'master' into dev
Merge branch 'dev' into refactor/layer
Adjust folder structure.
Pencil2D is split into 3 projects. Project 'pencil' is core lib, project 'app' is executable, project 'tests' is unit tests.
Merge branch 'dev' into refactor/layer

Conflicts:
	src/external/win32/win32.cpp
	src/graphics/bitmap/bitmapimage.h
	src/graphics/vector/vectorimage.cpp
	src/graphics/vector/vectorimage.h
	src/interface/backupelement.h
	src/interface/editor.cpp
	src/interface/editor.h
	src/interface/mainwindow2.cpp
	src/interface/mainwindow2.ui
	src/interface/scribblearea.cpp
	src/managers/layermanager.cpp
	src/structure/camera.cpp
	src/structure/camera.h
	src/structure/keyframe.cpp
	src/structure/keyframe.h
	src/structure/layer.cpp
	src/structure/layer.h
	src/structure/layerbitmap.cpp
	src/structure/layerbitmap.h
	src/structure/layercamera.cpp
	src/structure/layercamera.h
	src/structure/layerimage.cpp
	src/structure/layerimage.h
	src/structure/layersound.cpp
	src/structure/layersound.h
	src/structure/layervector.cpp
	src/structure/layervector.h
	src/structure/object.cpp
	src/util/util.h