timeline-tree: not using snapped values when testing if a trim is legal

Timline-tree only uses the given values, rather than the snapped values, for testing if a trim is legal. This can bypass some of the checks.

>>> tl = GES.Timeline.new_audio_video()
>>> layer = tl.append_layer()
>>> clip0 = GES.TestClip.new()
>>> clip1 = GES.TestClip.new()
>>> clip0.set_start(22)
True
>>> clip0.set_inpoint(10)
True
>>> clip0.set_duration(10)
True
>>> clip1 = GES.TestClip.new()
>>> clip1.set_duration(10)
True
>>> layer.add_clip(clip0)
True
>>> layer.add_clip(clip1)
True
>>> layer
<GES.Layer object at 0x7fae68c1c050 (GESLayer at 0x55752e696410)>
>>> layer.get_clips()
[testclip4 [0:00:00.000000000 (0:00:00.000000000) 0:00:00.000000010], testclip2 [0:00:00.000000022 (0:00:00.000000010) 0:00:00.000000010]]
>>> tl.set_snapping_distance(2)
>>> clip0.trim(12)
True
>>> layer.get_clips()  # in-point has underflowed
[testclip4 [0:00:00.000000000 (0:00:00.000000000) 0:00:00.000000010], testclip2 [0:00:00.000000010 (5124095:34:33.709551614) 0:00:00.000000022]]