expose button font size publicly

enable clip in window (this breaks titlebar though), need to fix
max sizing/layouting of the windows (options dialog), fill main menu
main buttons to the full layout now, all the same size
expose winContents in window, parent to that instead from now on i suppose

fix parenting of dialog, centering, and margins of window and titlebar
center options dialog on show
introduce OptionsDialogBackend, and hook optionsdialog qml into it

trigger finalizeSettings() (slot within backend, will take gathered
properties as they're changed and push them when the slot is called,
when the dialog is closed accepted)