removed apply button
bugfix: disable all material boxes ...

when the maximum number of extruders of all printers became smaller, not
all material combo boxes where disabled.
moved printer selection to SettingsEditor
implemented deletion of PrinterSettings
added implementation of create new printer setting
implemented g-code editing panel
moved panel creation from SettingsEditor to SettingsNode and ...

created AbstractSettingsPanel to hold implementation of panel layout
refactoring: pulled up common code for panels that ...

modify PrinterSettings: CustomGcodePanel, GeneralPrinterSettings,
ExtruderPanel
extruder panel layout
implemented extruder settings binding
added panels for print settings with icons, no functionality
implemented print settings selection panel and ...

added FilamentSettingsPanel (with icon and without functionality)
extracted common coding of CategorySelectionPanels
added Material selection panel

(undecided how to implement multiple current materials)
Got rid of last usage of DummySettingsPanel
split package ...gui.configuration
implemented layer settings panel
implemented infill settings panel
implemented speed settings panel
implemented skirt and brim settings panel
implemented support setting panel and a fill pattern control