Add preference for terminal windows buffer size (bug #41474) * QTerminal.cc (notice_settings): call new function SetScrollBufferSize with the related value from the settings * QTerminal.h: new purely virtual function SetScrollBufferSize * QUnixTerminalImpl.cpp (initialize): corrections of coding style; (setScrollBufferSize): implementation of new function; * QUnixTerminalImpl.h: new function setScrollBufferSize; * QWinTerminalImpl.cpp (class QConsolePrivate): new function (QConsolePrivate::setScrollBufferSize): implementation of new function; (QWinTerminalImpl::setScrollBufferSize): implementation of new function; * QWinTerminalImpl.h: new function setScrollBufferSize; * settings-dialog.cc (constructor): init spinbox with buffer size from settings; (write_changed_settings): write value of spinbox into settings file * settings-dialog.ui: new spinbox for terminals buffer size (terminal tab)