started writing a scale-bar overlay:
Using source and viewer transform to determine screen to pixel scale works.
Determining appropriate scale-bar length, label and units works.
Still missing is getting the VoxelDimensions from the Source.
Sources provide VoxelDimensions that are used in the scale bar
bugfix: when removing a source i < currentSourceIndex, decrement currentSourceIndex
Source.getSourceTransform() takes an AffineTransform3D argument which is set to the requested transform.
Old Source.getSourceTransform() is deprecated.
replaced all uses of deprecated getSourceTransform()
added tools to quickly create bounding box dialog, source, and overlay
Merge branch 'master' into scalebar

Conflicts:
	core/src/main/java/bdv/util/RealRandomAccessibleSource.java
typo
update name of shortcut properties file
add global Prefs that can be read from properties file.
add pref keys "show-scale-bar" and "scale-bar-color".
added useVolatileIfAvailable option to ViewerPanel
store transform used to render image with the rendered image for accurate overlay painting.