tray: better name for callbacks
tray: remove duplicated 'activate' signal handler

For the sake of convenience, everything is handled by the
'button-press-event' handler, since we are dealing with several mouse
buttons in the application.

Previously, left-clicking on the tray icon used to trigger both the
'button-press-event' and the 'activate' event, which actually tried to do
the same thing.
tray: simplify logic to decide either to hide or show the main window
tray: fix hiding/showing the main window even if it's not the active widget

If the main window is not the active widget (ie. if it doesn't have the
focus), it can now be hidden/shown by left-clicking on the tray icon.

Fix #43
tray: fix hiding/showing the main window even if it's not the active widget

Merge branch 'fix/43-show-hide-click-tray-window-not-active'