[3591] Add EditProject subtitle extension point

[wpiers]

Bug: #3591
Signed-off-by: William Piers [email protected]# Pull request template

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[sbegaudeau]

I'll rebase the PR on master to see its commit a bit more clearly

[sbegaudeau]

I'll add another small improvement within this PR to let contributors add such kind of extensions without flickering of the user interface.