This commit makes addition of keyframes possible.

Conflicts:
	pitivi/timeline/elements.py
	pitivi/timeline/timeline.py
timeline/elements: Edge keyframes can't have their timestamps changed.
timeline/elements: clamp timestamps and values.
timeline/{elements:timeline} : Unselect keyframes on dragEnd if needed.

	+ Get the pointer device in timeline.
	+ Check if it is on the keyframe at drag_end in elements.
timeline/elements: add a way to remove keyframes.

It's possible to remove keyframes now by holding CTRL and clicking a keyframe.
timeline/elements: hide keyframes when the element is not selected.
utils/pipeline: seekRelative(0) is a legitimate case.
timeline/elements: When keyframes are edited / added, don't react to

clicks on the stage.
timeline/elements: Moving a line now moves the adjacent keyframes.
clipproperties/widgets : switch to using a CheckButton.

	+ Also fix behaviour for hiding keyframes.
GstElementSettingsWidget: make widget insensitive when in keyframes mode.
timeline/elements: Add double support for keyframe removal.

	+ Also, don't unselect the clip when removing.
viewer: Don't try to seek the tmp pipeline before being in PAUSED.
timeline/elements, utils/widgets: reset keyframes curve properly.
timeline/elements: Don't overlap first and last keyframes.
clipproperties: remove keyframe curve when removing an effect.
utils/widgets: add binding to the bindings dictionnary

when adding keyframes for the first time.
timeline/elements: Add a menu that pops when the keyframe

is left-clicked, and gets hidden when the user clicks elsewhere
in the timeline.
timeline/elements : Update keyframes when duration or inpoint

change.