[Feature request] Allow only one instance setting

[subcor]

Since Pinta can handle several files opened at the same time having tabs it would be useful to have a setting that would force only one program window for convenience and resource usage (For example when launching from Windows Explorer with the "open with" function.)

[cameronwhite]

currently Pinta is launched with Gio.ApplicationFlags.NonUnique which is why multiple instances are allowed.

I don't have any objection to the setting, but there's probably a bit more work needed to make this work properly - see
https://bugs.launchpad.net/pinta/+bug/1909924 and e4ff9bd where there were issues with Pinta crashing on Linux before we started using the "non-unique" flag

[bv-saketha-rama]

can I work on this issue? pls assign it to me