world-dialog: explicitly grab focus so that the user may visualize it

When opening the dialog, the user can start typing and the search bar will be filled with text, however that is not shown as possible to the user - the search box has no "blue border".

By explicitly grabbing focus, the search entry becomes highlighted. However, that means the user can't press ESC to escape the dialog as he should be able to do. on_search_mode_notify is already called when the user presses ESC, so use that callback to check whether the user has searched anything. If he did not, then close the dialog.

I tried to find a way to just "enable" the highlight / blue border, but I failed in doing so. This ended up solving the problem though and as I'm not very familiar with gtk, I'd be very glad to hear alternatives! :^D

This closes #332.