Added files tree to GUI.
Added drag&drop support for files and folder.
Now GUI can process single file or files added by user from different
folders.
Fixed saving svgz as svg.
Icons updated.
Added preset description.
Added ungrouping of groups with only transform attribute and when all
children has transform attribute too.
Addec circle and ellipse replacing by use.
TinyXML2 updated to last version.
Added processing of rotate(<rotate-angle> [<cx> <cy>]) transform.
Added saving of mirror transform as scale transform, not matrix.