Fix: segfault when dbus server is not running

Upon an unsuccessful connection to the dbus server, it is not possible
to obtain response arguments. According to the Qt documentation it is
up to a programmer to check if the replay is valid.
Allow try icon customization

Check for "tinymount" icon in the themed icons set, and as a fall-back
use build-in one.
Merge pull request #13 from Arkq/master

Segfault fix + small improvement