Add support for custom app_id for window managers

[mkungla]

This PR adds an app_id field to App, allowing users to set a custom app ID via the new --app-id command-line argument. build_window_options now uses cx.app_id, defaulting to the release channel app ID if none is provided.

Why?

While there are other potential use cases for a custom app_id, I personally use this to make it easier to manage multiple org/project workspaces. Zed’s concepts of Workspaces and Projects can be a bit confusing, and support for multi-root workspaces is currently missing (see issues #9459, #15120, and likely others). There's also no way to open a workspace by ID from workspaces table in db.sqlite.

Instead of making big or breaking changes, this PR introduces a minimal but useful tweak to help mitigate the lack of more advanced workspace support.

For example, having follwing project structure :

org1
    ├── logo.png
    ├── project-1
    ├── project-2
    └── project-3

Then, using a custom .desktop entry, you can assign a unique app_id so that window managers group all projects from the same org together. This doesn’t fully solve the problem of managing complex/multi-root workspaces, but it does provide a small quality-of-life improvement when dealing with massive org codebases.

Example dev.zed.Zed-Org1.desktop entry

[Desktop Entry]
Version=1.0
Type=Application
Name=Org 1
GenericName=Text Editor
TryExec=zed
Exec=zed-editor --app-id dev.zed.Zed-Org1 /devel/org1
Icon=/devel/org1/logo.png
Categories=Workspaces;
Actions=Project1;Project2;Project3;

[Desktop Action Project1]
Exec=zed-editor --app-id dev.zed.Zed-Org1 /devel/org1/project-1
Name=Open a project 1

[Desktop Action Project2]
Exec=zed-editor --app-id dev.zed.Zed-Org1 /devel/org1/project-2
Name=Open a project 2

[Desktop Action Project3]
Exec=zed-editor --app-id dev.zed.Zed-Org1 /devel/org1/project-3
Name=Open a project 3

Release Notes:

• Add support for custom app_id for window managers and --app-id flag

[cla-bot]

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[mkungla]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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