Fix the sidebar popup menu to properly use the currently selected item
rather than the previous one.

Also change the hack used for the selection to be updated in the input
handlers (key and mouse button) to call the GtkTreeView's handler
manually rather than doing the actual job in IDLE callbacks for the
TreeView's handler to have run.
This prevents from having several different handlers but removes the
need to work in IDLE time and then possible have a lag, as well as
simplifying the code.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5630 ea778897-0a13-0410-b9d1-a72fbfd435f5