NLA: Always Show All Strips

Currently, it's difficult to use the NLA system for users who are only
interested in using it as animation layers.

Entering tweak mode hides strips which are not evaluated. If the user
wanted to edit a different strip, they must exit tweak mode, look for
the strip, select it then re-enter tweak mode.

Solution:
All strips are always shown. The user can now see the next strip they
want to start editing.

Reviewed By: Sybren, Sebastian Parborg

Differential Revision: http://developer.blender.org/D7600

Fix: NLA, transition to Meta Strip uses wrong time

Copy correct time to temporary strips used for evaluation of NLA
Transition strips.

When a transition is placed next to a meta strip, the transition would
use the wrong strip time, using the time of the meta strip instead of
its own.

Reviewed by: sybren

Differential Revision: https://developer.blender.org/D8287

Fix T63675: Keying outside NLA strip not possible

Allow adding keys outside of the NLA strip, when Sync Length is
activated. Before this, an animator would have to exit tweak mode,
adjust the strip length, then enter tweak mode again.

Now the animator can freely insert and modify keys outside of the strip
bounds. They will never have to touch the strip bounds directly.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D7533

NLA: Fix strip Sync Length moving animation data

Change the way Sync Length works so that it keeps keyframes at the same
key as before the sync. Now when the user exits tweak mode, the overall
NLA result is unchanged.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D7602

RNA: expose the pin flag for AnimData and ActionGroup