mpris: Only import 'indicate' if $DISPLAY is set

This avoids printing of the following error message:

    ImportError: could not import gtk.gdk

Fixes #311
mpris: Disable extension if no $DISPLAY set

We cannot connect to the D-Bus session bus if there is no X11 $DISPLAY
available.
Merge pull request #407 from jodal/feature/mpris-needs-display

Turn off MPRIS if no $DISPLAY available
ext: Move ext helper functions from 'main' to 'ext'
log: Remove system info available in --list-deps
docs: Add empty troubleshooting page
docs: Rename 'changes' to 'changelog'
docs: Reorganize frontpage sections
docs: Add empty 'contributing' page
docs: Tweak authors page
docs: Reduce depth of development ToC
config: Convert mopidy.config to module
config: Remove register schema as it is not used
docs: Updated intro
config: Move everything to mopidy.config sub-modules
config: Review fixes
docs: Fix wording
docs: Add extension registry
docs: Move scrobbler docs to new ext registry
config: Move load, validate and overrides code into mopidy.config