[Developer] Projects should remember which files were open and re-open them

**Is your feature request related to a problem? Please describe.**
When switching between different projects in Keyman Developer, it makes no sense to leave the current files open.

**Describe the solution you'd like**
Current files should be saved (with prompt), closed, and then the new project's last open files should be opened.

**Product context**
- OS: Windows

It may be also good to consider whether or not it makes sense to have untitled projects in Keyman Developer 13.