Blender Git Loki

Keymap: 'M' for edit-mesh merge menu, 'Alt-M' for split menu

As the 'M' key is free, it's convenient to use for the merge menu,
especially since this contains "Merge by Distance",
a frequently used action.

Use 'Alt-M' for a new split menu, following our convention of Alt being
used for opposite functionality.

Also move merge/split menu's into the "Mesh" menu as neither operate
solely on a single mesh element type.