Reliably open documents from sidebar

[jeremypw]

Fixes #1403
Fixes #972

Using the item-selected signal from the SourceList to open documents causes problems when documents are removed for any reason because this causes unwanted documents to selected and opened. In master this is fixed by a hack involving some select signals being ignored but this is not reliable. Quite often clicking on an item in the sidebar fails to open it because the signal is falsely ignored.

Instead, this PR uses item activation to trigger document opening/tab selection. To maintain the same single click behaviour the SourceList is modified so activate_on_single_click is accessible.

[colinkiama]

Works great!

[colinkiama]

@elementary/maintainers seems like something is wrong with the CI checks in this pull request.

All the checks have already passed but GitHub insists that there's one more required lint check remaining.

[jeremypw]

Seems the linting check has stalled for some reason.

[jeremypw]

Hmm, after removing automerge and re-running CI it is now failing on Flatpak build. It seems to be looking for Gtk.SourceView. Still not reporting the lint result although it passed.

Edit: I confused SourceView and SourceList we still need Gtk.SourceView.

[jeremypw]

Looks like the gitlab server is not available atm so SourceView source code not available for building the Flatpak.

[jeremypw]

@danirabbit Any idea what is blocking this from merging? CI seems to be passing. Is it OK to bypass the requirements?

[danirabbit]

Sorry about that. Made a mistake migrating to the new branch protection rulesets