Use uniform "Combine" name for Combine / Merge operation

[jidanni]

Its shortcut is "C".
In the Help manual it is called "Combine (merge) selected features".
In the success / error messages it is called "Merge".

I would just call it "Combine" everywhere.

[matkoniecz]

In the Help manual it is called "Combine (merge) selected features".

that allows finding it with search/grep etc, so seems fine to me

[tyrasd]

I would say that Merge is the more intuitive to understand term, that's why the messages use merge everywhere… Maybe we can just rename the entry in the keyboard shortcuts list to

C Merge (combine)