Pyslvs v20.06.0

Features:

• Polar plotting function and joint viewer.
• Animation function for the paths of inputs page.
• "Append path" function for the path of dimensional synthesis page.
• Remove deprecated "show solutions" buttons.
• Fix GUI crash on AppImage. (github issue #34)

Development:

• Support MSVC versions 1910~1916 and 1920~1926.
• Improvement of UI and QRC file generating.
• Annotated object attributes are placed in class definition uniformly instead of method blocks.
• Use immutable "Mapping" replace "Dict" when no need to change the content.