StringElement bugfix (Horizontal text alignment)

Bug description:
Multiple StringElements in the same View (some with values set, some without values/only with caption)
Set the alignment of the string element without values to "UITextAlignment.Center"
The alignment of the center elements switches randomly between "left" and "center" at each "ReloadData".

=> reason: if a value is set for a string element, the reuse identifier has been wrong (differed from the reuse identifier which is used to find the element)
Merge pull request #195 from Boardwish/master

StringElement bugfix (Horizontal text alignment)