Blender Git Loki

Transform manipulator: Allow first clicking Shift before selecting axis

Avoids possible jumps when one is trying to do some really preciese tweak.

Quite striaghtforward change for mouse input initialization: take Shift
state into account. However, this will interfere with the axis exclusion
which is currently also uses Shift (the feature to move something in a
plane which doesn't have selected axis). This is probably not so commonly
used feature (nobody in the studio even knew of it) and the only downside
now would be that such a constrainted movement will become accurate by
default. That's easy to deal from user side by just unholding Shift key.

Reviewers: brecht, mont29, Severin

Differential Revision: https://developer.blender.org/D2418