Material editor crashes and other bugs

Some bugs have been found while reworking Material Editor:

When a different material shall be chosen for a particle, BornAgain crashes

Before the crash, this dialog appears:

.

Then the application exits.

This has been introduced at some point since the last release (not present in BornAgain version 1.18)

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When removing an already used material, BornAgain crashes

In the material editor, a material can be removed despite it is used in a particle.

Later, when running the simulation, BornAgain crashes.

On other occasions errors occur (e.g. when showing the 3D view - "material not found")

expected: material can't be removed, a hint is shown

BornAgain version 1.18

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When the color selection dialog is canceled, the original color is lost

expected: the color stays the same as before

BornAgain version 1.18

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Wrong tooltip on color selection button:

expected: "Choose color" or similar

BornAgain version 1.18

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After adding or cloning a material, the new one is not selected

expected: it should be selected for a better workflow.

BornAgain version 1.18