Release v2.0.0

• Supports Qt6 in addition to Qt5 (co-installable)
• Buildsystem: increase CMake min version to 3.16.0
• Buildsystem: add uninstall target
• Buildsystem: generate and install kdsme-version.h
• Use official Graphviz from upstream with -DKDSME_INTERNAL_GRAPHVIZ=True
• Fix build with more recent graphviz versions
• Buildsystem: new Option BUILD_SHARED_LIBS to select between shared or static libraries
• Rename the following CMake options:
  BUILD_DOCS -> KDSME_DOCS
  BUILD_EXAMPLES -> KDSME_EXAMPLES
  BUILD_QT6 -> KDSME_QT6
  WITH_STATIC_GRAPHVIZ -> KDSME_STATIC_GRAPHVIZ
• Use standard cmake options instead of custom ones:
  BUILD_TESTS -> BUILD_TESTING