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
wayland: Don't crash on startup.

Check for X11 / Wayland in GDK.
sink: use glimagesink