Pyslvs v20.04.0

Features:

• Fix relocate function and increase accuracy of its spinbox.
• The zoom bar has been isolated.
• Add grid mode toggle button at the main window.
• Remove not useful functions.
• Add "report issue" action on the main menubar.
• UI improvement for "inputs" page.

Development:

• Use Matplotlib instead of PyQtChart.
• Fix the error of auto preview path.
• Fix the bug of topological algorithm. (github issue #33)