Reliably open documents from sidebar

[jeremypw]

Fixes #1403

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.