Load filters calling controlling code

Should use dependency injection where practical.
Move Mounter.* methods to udiskie.mount.* module level functions

This is more natural since these functions have no side effects inside
the scope of the Mounter class. The Mounter class is kept however as a
utility to remember bus, prompt and filter.

For useful diffstat compare with

    git --ignore-all-space
Relocate udiskie.umount.* to udiskie.mount.*

Keep udiskie.umount.* for backward compatibility.
Add mount/mount_holder functions to make API consistent
Merge branch 'general-mount-utility' into separate-mvc-components
Add slightly more elaborate docstring to udiskie.device
Apply python3 compatibility patches