Restore "import shared" to helper_inbox

Commit 5b23d9 removed the line "import shared" from helper_inbox. Almost
all of what shared was used for became covered by helper_sql. But, shared
still needs to be imported because there is still one line that uses
shared:
9:    shared.UISignalQueue.put(('removeInboxRowByMsgid',msgid))
Merge pull request #471 from Eylrid/master

Restore "import shared" to helper_inbox