gdk: quartz: Avoid page up/down getting filtered by gtk_im_context_filter_keypress()
For page up/down events (Fn+up_arrow and Fn+down_arrow on macOS) gtk_im_context_filter_keypress() currently returns TRUE when im-quartz is used. This means these events get removed when this function is used (happens e.g. with the Scintilla text editing library).
Adding scrollPageDown: and scrollPageUp: into GdkQuartzView seems to resolve the issue as these seem to get called instead of the already present pageUp: and pageDown:.