* Fix DynamicTextSpanner left alignment.

[n . puttock]

Reviewers: Neil Puttock,

Description:
* Fix DynamicTextSpanner left alignment.
When a DynamicTextSpanner follows a DynamicScript, padding is
required to ensure the 'left 'text doesn't collide with the
dynamic sign.  Unfortunately, this default value skews the
spanner's left bound to the right when no dynamic script is present.

This patch caters for both situations by resetting 'left 'padding
as required, adding extra padding read from the script property
'right-padding.

Additionally, two minor changes have been made to 'bound-details:

- vertical alignment for the left stencil is improved by using
'stencil-align-dir-y
instead of 'stencil-offset;
- some padding is inserted between the stencil and the line, matching
the right bound.

Please review this at http://codereview.appspot.com/7763

Affected files:
  A input/regression/dynamics-text-left-text-alignment.ly
  M input/regression/dynamics-text-spanner-abs-dynamic.ly
  M lily/new-dynamic-engraver.cc
  M lily/script-interface.cc
  M scm/define-grobs.scm