Add the "Copy Selection" context menu item after checking for chat items

When right clicking a channel, Quassel will drop all other items from
the context menu.
We work around this by checking for a selection after everything else
has been added to the menu, and make sure to add the Copy Selection item
at the top.

Fixes issues #1031 and #1048
Fix context menu not being restored properly

When showing a context menu on a clickable, we used to hide existing
context menu item to avoid confusing the user. However, those actions
were never unhidden again. This resulted in the context menu not showing
most options ever again in subsequent invocations.

Instead of hiding the actions, we now just clear the menu.
Add separator after "Copy Selection" context menu

This went away with the commit fixing the menu for selections on
clickables, and is now being brought back.
Update AboutDlg

Some new contributors deserve mentioning.
Update ChangeLog