Return to Metal as default Look and Feel

[WolframPfeifer]

PR #3599 switched the default look and feel to "FlatLaF Light", which introduced a few deteriorations in usability (see #3599 (comment)). While some points were fixed by #3645, we decided in the KaKeY meeting today that for the moment we want to stick to "Metal" (standard of Java, our previous LaF) as default, since a few things are not yet fine-tuned for FlatLaF (contrast, size of buttons, background of components, ...).

In addition, we disable the setting "defaultLookAndFellDecorated" by default, since it makes the window not responding to resize/drag operations with some themes (at least Metal and Nimbus). There is now a checkbox for that option in the settings dialog as well.

Intended Change

• makes the "Metal" look and feel the default (Java standard LaF)
• disables "defaultLookAndFeelDecorated" by default
• adds a checkbox for "defaultLookAndFeelDecorated"

Type of pull request

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• There are changes to the (Java) code

Ensuring quality

• I made sure that introduced/changed code is well documented (javadoc and inline comments).
• I made sure that new/changed end-user features are well documented (tooltips in the GUI).
• I have tested the feature as follows: by manual inspection with the GUI (switching back and forth the new checkbox and the LaF)

The contributions within this pull request are licensed under GPLv2 (only) for inclusion in KeY.