Fix unicode problem with non ascii character in on_transfer_started

Traceback (most recent call last):
  File "/usr/bin/blueman-sendto", line 164, in on_transfer_started
    "4": "∞"}
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 0: ordinal not in range(128)
Fix getting style with Gtk3