Updated Brazilian Portuguese translation
previewers: Only check messages from the level element to find infos about level

We were parsing each and everyt message wish was useless
previewers: Make sure only 1 audio and 1 video pipeline run concurrently

In order to be able to perform well in case we are trying to generate
thumbnails and waves for many clips at the same time we need to manage
the ressource in a concervative way.

The idea in this commit is that we have an object that implements a
queue of preview generator and control their execution. For that we also
introduce a PreviewGenerator interface that must be implented by any
class that generates previews.

+ We are now able to recover from errors in the waveform previewer, if
we get an error on the bus, we stop the pipeline, and add the
PreviewGenerator again to the PreviewGeneratorManager so it is tryed again latter,
the second time there will be no rate modulation making it less error
prone, we abandone after 2 times (1 with rate modulation, the other
without)

+ The thumbnail generation stategy has been sensibly modified so that
the thumbnails are generated *only* for the visible area, not the whole
clip. This permit us to be able to get thumbnails for visible parts of
all clips even though we generate thumbnails for 1 clip after the other.
previewers: don't uselessly decode streams.
timeline: Don't Gtk.main_quit when not run in standalone mode.
previewers: fix bad conflict resolution.
timeline/elements: Cleanup and document code
timeline/elements: Improve the appearance and visibility of the keyframe curves

- Use a better default color (Tango yellow) for the lines
- Draw a "shadow" above and below the lines
- Improve the lines' height calculations by standardizing on keyframe heights
Standardize timeline colors with constants and fix the ruler's bg color

Fixes bug #704686
timeline: Fix insertion of image files with drag and drop
utils/timeline: Adds a "ClipEdited" undoable action.

	+ And use it in EditingContext
timeline/timeline.py: commit to action_log after adding a clip.

Always finish what you start.
Also, TODO add a "ClipAdded" UndoableAction.
Added Greek translation of help
timeline: Make scrolling with the mouse wheel on the ruler work again
timeline: Add some initial debugging statements and cleanup code
autoaligner: Make the auto-alignment feature slightly more robust

- Don't stay stuck with a modal progress window when something fails
- Remove the check for numpy, as it is now a hard dependency for waveforms
- Use a larger default window width (like the render progress dialog)
- Remove the window title icon and give the window a proper modal look
Make the import filechooser dialog modal so that it works in fullscreen
project: Prevent the backup restore dialog from being titled "pitivi"
timeline: Resurrect the "Save screenshot" feature

It was collateral damage from the Clutter timeline port.
This brings it back from how it was in commit 6211f3a3e9.
pitivi-git-environment: Add more workarounds to prevent blocking builds

These are only temporary workarounds. GStreamer folks don't seem to be
in a hurry to fix those minor issues and life is too short to keep wasting time
manually resetting my checkouts to prevent the build script from failing.