gtk+ 3.18.6 release.

Bugs fixed:

• 539944 Add GtkScaleButton API so struct fields can be marked as private
• 732742 Infinite recursion on GdkDevice disposal
• 735847 move animatable cursors' current frame info into GdkCursor
• 746745 Window suddenly always on top
• 747295 shared memory leak when creating/destroying widgets
• 748904 wayland: map real modifiers to virtual modifiers
• 757358 single touch devices are not detected as touch devices
• 758025 cursor flickering
• 758072 [Wayland] additional mouse buttons don't work
• 758175 Fix character entity
• 758327 gnome-disks segfault in gdkwayland get_popup_parent - dereference...
• 758367 GTK+ 3 does not respect bold in org.gnome.desktop.interface font-name
• 758407 Could not send the search request \ GDBus.Error:org.freedesktop...
• 758483 GDK W32: Incorrectly uses SetWindowLong() to set/unset WS_EX_TOPMOST
• 758484 GDK W32: Some calls to W32 routines are rather unclear
• 758660 Copy and paste doesn't work in wayland session
• 758661 every second right-click to window header is ignored (CSD-related?)
• 758698 Bug backward search + case insensitive + preceding multi-byte character
• 758790 A few stylistic icon-browser patches