[buttontranslator] fix keymap overrides not working

I found an issue with buttontranslator that I introduced in https://github.com/xbmc/xbmc/pull/5364 causing keymap overrides to no longer work.
This is due to the line
``` C++ 
(*w)[windowID].insert(map.begin(), map.end());
```
which merges two maps, but favors the original entries for the keys contained in both maps, instead of overriding them with the merge map.