ephy-window: Ctrl-Shift-o now toggles overview

Pressing Ctrl-Shift-o opens the tab overview. However, pressing it again while inside the overview does not close it. This is inconsistent and feels unnatural.

This fix changes the ephy_window_open_tab_overview() function to ephy_window_toggle_tab_overview(). It now acts as a toggle, so it can both close and open the overview.

This does cause a problem when holding down Ctrl-Shift-o (the screen flashes between overview open and closed), but it could be easily resolved by mapping toggling the overview to when the keyboard shortcut is released, not right when it is pressed.

Fixes: #2147 (closed)