Fix warnings on OSX 10.10.

Fix warnings on OSX 10.10.

* nsfns.m (MODAL_OK_RESPONSE): New define for different OSX versions.
(Fns_read_file_name): Check against MODAL_OK_RESPONSE.
(compute_tip_xy): Use convertRectToScreen for OSX >= 10.7

* nsmenu.m (initWithContentRect:styleMask:backing:defer:)
* nsimage.m (allocInitFromFile, setPixmapData): Only call
setScalesWhenResized for OSX < 10.6.

* nsterm.h (EmacsScroller): Declare scrollerWidth.

* nsterm.m (ns_copy_bits): New function that does not use deprecated
NSCopyBits.
(ns_scroll_run, ns_shift_glyphs_for_insert): Call ns_copy_bits.
(runAlertPanel): New function.
(applicationShouldTerminate:): Call runAlertPanel.
(initFrameFromEmacs, toggleFullScreen:): Only call useOptimizedDrawing
for OSX < 10.10.
(initFrameFromEmacs:): Only call allocateGState for OSX < 10.10.
(windowWillUseStandardFrame:defaultFrame:): Cast arg to abs to int.
(draggingEntered:): Returns NSDragOperation.
(scrollerWidth): Use scrollerWidthForControlSize for OSX >= 10.7.