custom style of dock widget title bars depending on focus (bug #43837) * octave-dock-widget.cc (ctor): connect active-dock-changed signal with the related new slot; (make_widget): prevent warning message on unused variable; (set_style): new function styling the title bar depending on activity; (handle_settings): get colors for custom style from settings file; (handle_active_dock_changed): new slot for signal from main window settings the custom style depending on changed activity * octave-dock-widget.h: new slot handle_active_dock_changed, new function set_style, new class variables for custom colors * settings-dialog.cc (ctor): init color pickers for back- and foreground color of title bars of active dock widgets from settings file; (write_changesd_settings): write back- and foreground color of title bars into settings file * settings-dialog.h: new class variables for active back- and foreground colors * settings-dialog.ui: new color pickers for active back- and foreground colors

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