Be able to rename notes

[hakanu]

One implementation may be like:

1. Add a button in tab view after opening the note with a name "Rename"
2. Open a rename modal
3. Take new name from that modal which has old name prepopulated.
4. Add a new endpoint in server.py as /api/file_rename which takes path and new name.
5. Returns success
6. Sidebar refreshes
7. Open tab path is modified with new path which contains new name

[clach04]

One of the items in the readme has a note that users are in control of filenames and that files are never auto named. Is this a set-in-stone feature? If not then naming the file based on the first line (as https://github.com/pbek/QOwnNotes and Tombo does) is another possible alternative.