app: get rid of server grabs for almost all canvas interaction

Because it's generally the right thing to do, and server grabs broke
badly with input devices / client side windows.

gimpdisplayshell-grab.c: change logic to only server-grab if an event
is passed to the pointer grab/ungrab functions, but always use
gtk_grab_add/remove() which is sufficient in most cases.

gimpdisplayshell-tool-events.c: have the grab functions grab the
server only for space-bar scrolling and do all tool interaction,
including ruler clicks, with gtk_grab_add/remove(). Refactor things
a bit to also use the grab API for button-2 scrolling.

gimpdeviceinfo-coords.c: transform the event's coords to the canvas'
coordinate system, they might come from a ruler now.

This fixes the following bugs:

Bug 645315 - gimp_display_shell_pointer_grab: gdk_pointer_grab failed...
Bug 644351 - Gimp misses some strokes especially when drawing fast
Bug 645747 - Gimp is now unusable on xfce4