[CenterOfMass] Added support to enter a mass and other improvements

[s-quirin]

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Changelog:

• New: Masses are editable (a highly requested feature)
• New: Highlights the solid you are working on
• New: No duplicate entries when container and content selected concurrently
• New: Pie charts show density relation in combo box
• New: Legibility of text on colored combo box with WCAG21 1.4.6 Contrast (Enhanced) conformity
• Fix: A Part used as a container for meshes (e.g. .stl's) is recognized correctly
• Fix: Fixes for error and language handling, material editing, combo box and GUI size adjustment

[galou]

Thanks for your contribution!