plug-ins: differentiate a delay before the area/window selection...

... and a delay before the screenshot.
Until now, there was only delay before selection, which I changed in
commits d9cd4b61 and 614bcf6d. Actually a delay before selection may
also be useful, for instance when you use a tablet without keyboard (no
alt-tab possible) and the window/area you wish to capture is behind GIMP
window. Then you'd want to interact with the desktop with the pointer
before the cursor changes for selection interaction.
I add some logics so that the selection delay doesn't show when it is
unecessary (for instance for full-screen screenshot, or when the window
screenshot is based on the active window, not click selection, like with
GNOME shell API).