GUI: Do not enable per-message actions on startup

...or on network reconnect. These actions all depend on the set of
selected messages. As a result of more or less non-specific slot
execution order, the GUI items were inadvertently enabled even though no
messages were actually selected.

BUG: 354760
Change-Id: I0b5ce16e93c8ca431b5a6853ea4d30927b43007a