keysync-gui: detect if pymtp is available, and use it if its there

Before, pymtp was only used on Mac OS X, which has no MTP support. But it
turns out to be useful on GNU/Linux as well.  Sometimes gvfs FUSE mounting
fails to be available, but then the MTP send_file_from_file still works.

refs #1939
support pymtp v0.0.6, which has essential bug fixes

refs #1939
set version to 0.2.0beta1 in prep for upcoming release