entrycompletion: Don't reconnect signals all the time

entrycompletion: Don't reconnect signals all the time

We block signal handlers areound GtkEntry signal emission and if those
signals get used to call functions on the completion that cause a
reconnection of the signals, then the reconnected signals will not be
blocked anymore (so they might get emitted?) and unblocking the old
signal id will later cause warnings.

Fixes spurious warnings in gtk/tests/filechooser tests.