main: Make search entry keyboard navigable
Add a dropdown button next to the search entry to select the search source. The button is reachable through the keyboard via tab navigation. The label indicating the selected source is just a label and not clickable anymore.
Search entry can be refocused through tab navigation.
Closes: #71 (closed)
Additional info: Removed a GtkBox wrapper around TotemSearchEntry, which is a GtkBox itself. I guess this was the reason it could not be refocused – and/or something with the "can-focus" property.