Add "Update Submodule" to the context menu of submodules in the "Files" list.

**Is your feature request related to a problem? Please describe.**
If I want to update a submodule I have to open the submodules list and press "u" on it there. It would save me the extra steps if I could just select the submodule in the Files list instead and do it there.

**Describe the solution you'd like**
Add "Update Submodule" to the context menu of submodules in the "Files" list.

**Describe alternatives you've considered**
I can use the Submodules list but this would be quicker.