Project database allows for connection from multiple Python instances

[pedrocamargo]

Purpose of use
Prevent one Python instance from opening a project that is already open in another Python instance

Is your feature request related to a problem? Please describe.
There is potential for a problem, so better do it

Describe the solution you'd like
There are two possible options here:

1. Create a token file somewhere in the system (or in the project folder) to signalize that the project is open (and delete when it is done)
2. Create one record in the about table signalizing that the project is open

Describe alternatives you've considered

• Using https://sqlite.org/pragma.html#pragma_locking_mode would be ideal, but it would break the "get connection" that is spread throughout the code