fix handling of scroll by page GTK setting

Closes #541 (closed)

gtk-primary-button-warps-slider can be set to 0 to revert GTK to the previous default to scroll by page. When doing so totem cb for the gtkrange value_changed signal is not called in between this gtkrange button pressed callback and button released callback. Thus the bug, ie the slider does not move at all.

Fix by overloading this setting when the totem slider is clicked. That is hardcoding to "scroll to the location of the click" on the slider whatever the GTK setting value is.