Changed text references from GTK+ to GTK, removed trailing white spaces.

Hi, I changed the references of GTK+ to GTK and stripped the trailing white spaces, in files ending with .page in platform-overview folder. For quick info on what files changed, have a look at the following text:

gnome-devel-docs/platform-overview/C/dev-help-write.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/dev-launching-icons.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/overview-net.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/tech-gtk.page :: changed to GTK
gnome-devel-docs/platform-overview/C/tech-network-manager.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/tech-pango.page :: changed to GTK
gnome-devel-docs/platform-overview/C/overview-io.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/tech-atk.page :: changed to GTK
gnome-devel-docs/platform-overview/C/overview-ui.page :: changed to GTK, trimmed trailing spaces
gnome-devel-docs/platform-overview/C/tech-cairo.page :: changed to GTK
gnome-devel-docs/platform-overview/C/overview-settings.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/overview-communication.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/dev-launching-mime.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/overview-media.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/tech.page :: trimmed trailing spaces, stripped right spaces
gnome-devel-docs/platform-overview/C/tech-spell-checking.page :: changed to GTK
gnome-devel-docs/platform-overview/C/dev-launching-startupnotify.page :: changed to GTK, trimmed trailing spaces
gnome-devel-docs/platform-overview/C/dev-translate-build.page :: stripped right spaces
gnome-devel-docs/platform-overview/C/tech-gdk.page :: changed to GTK
gnome-devel-docs/platform-overview/C/tech-gda.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/dev-help-build.page :: trimmed trailing spaces, stripped right spaces
gnome-devel-docs/platform-overview/C/tech-clutter.page :: changed to GTK
gnome-devel-docs/platform-overview/C/dev-launching-desktop.page :: trimmed trailing spaces
gnome-devel-docs/platform-overview/C/tech-webkit.page :: changed to GTK