Suggestion/feedback: better visibility for opening/saving project

After using Workbench for small quick tests and demos, I decided to test and prototype a more complex project to explore the GNOME APIs (it's great tool, thanks for making this, I'm really glad this software exists!).

Since I didn't see a way to save the project (first I tried to hit Ctrl+S, but I also noticed there's no option in the main menu[^1]), I ended up keeping Workbench running, never turning my computer off. I also kept copying and pasting the source code of the different files into a separate text editor out of caution, to make sure I wouldn't loose anything.

[^1]: Also I thought the "Open project..." menu item would take me back to the Demos list in the welcome screen.

After saving a backup once again I felt confident enough to close the program, and to my surprise I saw a dialog offering to save the project!

I was glad this functionality exists, but I wish it didn't hide itself like that! Since there's no save shortcut (a standard HIG convention), and no "Save project" item in the main menu, I thought there was no such feature, and avoided closing the app altogether, fearing I'd loose my prototype.

After opening Workbench again, another surprise: there was no way to open a saved project. Only after choosing one of the demo projects I got a way to select the "Open project..." menu item.

So, to summarize, two suggestions:

• Include "Save project" as an item in the main menu, following HIG's Ctrl+S keybindings.
• Include an "Open" button in the Header bar of the Welcome screen, allowing to open a saved project without having to open a random demo project first.

Appendix 1: Save dialog mock

I made a mock showing the save dialog. It's really just the existing one minus the "Discard" button + unsaved messaging:

Appendix 2: Open button in Welcome screen

Following the HIG suggestion of avoiding text-only buttons in header bars, I included a document-open-symbolic icon.

Start from the Welcome screen, the main menu, and the existing project save dialog described in the issue. Trace how projects are opened and how the save dialog is reached, then add visible Save and Open entry points with the requested Ctrl+S behavior; done means saved projects can be opened from the Welcome screen without selecting a demo first.