spu(audiobargraph_v,logo,marq,rss): position handling fixes

This fixes some issues with positioning option handling for { audiobargraph_v, logo, marq, and rss } spu plugins.

The initial 'bad default' fix is necessary, as mentioned in !49 (merged) because it results in buggy behaviour wrt. saving preferences, and is a very user visible problem within a new prefs interface I'll be submitting later.

The other fixes came about through evaluating how to correctly fix that.

I made some string changes in the first commit, but I feel that they are beneficial in significantly clarification. Note that the "X offset" and "Y offset" strings replacing "X coordinate" and "Y coordinate" are not new, they exist in the other modules, so we actually drop two for those.