Create multipolygon function: enable tags moving

[akks]

We have "create multipolygon" function in core but it does not add any tags to multipolygon and is inconvenient comparing to plugins.

Most functional plugin for relations is Zverik's Reltoolbox. Its multipolygon creation function is flexible and convenient.

I was asked at the forum to add tag moving functionality to standard action. I extracted tag changing function from Zverikk's action (which was long time ago copied from removeTagsFromInnerWays), leaving all advanced functionality (boundary management, multiple rings) only in plugin.

Here is the patch to review and compiled vesrion on DropBox to test:
​http://dl.dropbox.com/u/63393258/josm-custom-createrelation.jar