Disable File->Save All when there are no buffers to save

[renejsum]

Check for existing issues

• Completed

Describe the feature

A feature I appreciate in VSCode is that File->Save All is greyed out if all buffers are saved.

Environment

Zed: v0.160.7 (Zed)
OS: macOS 12.7.6
Memory: 64 GiB
Architecture: x86_64

If applicable, add mockups / screenshots to help present your vision of the feature

Coming from VSCode where it looks like this (1)
But in Zed if all buffers are saved, I can still click Save All.

Which is nice to check before doing a code reload in docker compose, which takes several seconds

[notpeter]

We currently do very little updating of MacOS menu items dynamically at runtime.

It appears VSCode disables the following menu items:

• File->Save All (this request)
• Go->Back/Forward
• Go->Last Edit Location (cmd-k, cmd-q)

Of note, VSCode does not disable these:

• File->Save, File->Revert File
• Edit->Undo, Edit->Redo, Edit->Find
• Go->Next Problem, Go->Previous Problem
• Splits, etc.