preview-command: Small fix in includepath
preview-modes: Add 'disable point-and-click'
preview-modes: oops. Revert second-to-last commit

I don't know why this is necessary ...
move musicxml export to ly.musicxml
Merge pull request #257 from uliska/no-point-and-click

Preview-modes: Add 'Disable point-and-click'
Merge branch 'master' of github.com:wbsoft/frescobaldi
correct order
Preview-mode: Refactor UI using loops

Don't have to explicitly create and handle each checkbox
individually. Now we have a dictionary of modes that can be
processed through loops
Separate Point-and-click from Preview modes in Engrave Custom
preview-mode: Improve coloring of voices

Now also manual stems benefit from the >=2.17.6 improvement.
They use \temporary\override then, so after reverting the
stem direction to neutral it won't revert to black but to the
previously active color.

As the other colorings (slurs, dynamics) don't interfere
with the coloring of voices I consider this issue fixed
with this commit.
preview-modes: Include lilypond-version-predicates

While this isn't needed currently it is a nicer and more
comprehensive file that may prove useful later
Merge pull request #260 from uliska/preview-mode-refactoring

Engrave Custom: Separate P-A-C from Preview Modes
dont assign translated strings to a variable on import time

they wont be translated when the user changes the language.
Instead, make them callable to the translation is done on request.
VCSRepo: Add abstract fetch() method
Make _run_git_command() more flexible (error handling)

As it seems Git commands are quite 'versatile' with regards
to their output on stdout and stderr ;-)
Therefore it seems better not to raise exceptions in the
basic _run method, even if that means more and some
redundant code in the higher-level caller functions.

_run_git_command() now returns a tuple of
- the stdout result (formatted as a list of strings)
- the stderr output.

Calling functions should handle the presence of 'error'
themselves, deciding whether to raise an exception or not.
VCS: Cosmetic touch to git checkout
VCS: Implement git fetch

Currently only works when the user has configured git
so it doesn't need interaction (e.g. passwords) when
communicating with the remote server.

Simply passes stderr output to the caller, so the calling function
(e.g. in the GUI) has to determine if an 'error' is regular info
output or indicates an exception
VCS: Add differing_commits() method

Compare local with (already fetched) remote branches.
Return
- remote_branches (that aren't present locally)
- local_branches (that aren't present remotely)

Can be used to determine the need of updates
VCS: Add app_repo.update() method

- Fetches from wbsoft/frescobaldi
- Determines if there are fast-forwardable updates
  (raises an exception if there are non-fast-forwardable updates)
- Switches to master
  (currently raises an exception if working tree isn't clean.
   Should this stash instead and reapply stash later?)
- performs fast-forward
- Switches back to previous branch
  (GUI should ask what user wants:
  - checkout and restart to master
  - continue current working session
  - try to rebase current branch to master and restart)
- returns a list of the commit messages of the applied updates
Fix import in vcs.menu