Naming inconsistency of Merging Posts / Topics in MCP

While working on the German translation, we came up with some problems with the merging of topics and posts.

1. MERGE_POSTS is defined in both common.php and mcp.php
The language variable "MERGE_POSTS" (Merge posts) is defined in common.php and in mcp.php. I don't see a reason why there should be both variables present.

2. The terminology is confusing
The quick-mod tools include two options "Merge posts" and "Merge topics". When calling the first of them (Merge posts), the secondary register in the MCP is titled "Merge topic". This is confusing to the user since "Merge topic" is a different action in the MCP.