Add label offset to styles

[tazle]

Currently it's not possible to specify label offset for styles. This is problematic when visualizing point features, since the text is drawn on the point, and will usually overlap any geraphical visualization on the point itself.

Looking at the code of the three renderers, adding offset should be simple. Just read the X and Y offset from the symbolizer, and add to the calculated screen coordinates.

I'm willing to write a sample implementation if there is no opposition to such feature in principle.

[madair]

partial solution in #1083

[jlacroix]

new patch using stylemap option and with an example

[jlacroix]

The first patch was using a layer option. After some discussion we agreed to use a StyleMap option instead. labelOffset2.patch also include an example of the functionality.

Marking for review

[ahocevar]

(In [9755]) added labelXOffset and labelYOffset symbolizer properties. Thanks madair for the original patch, and thanks jlacroix for turning it into symbolizer properties so nicely. p=jlacroix, r=me (fixes #2312)