pitivi: Clear the timeline when closing projects

Timeline was not cleared when closing the project / creating a new
project

https://github.com/pitivi/pitivi/issues/103
project: Add some debug infos
dogtail tests: Don't let the "project" test pollute /tmp
dogtail tests: Ensure the media library's iconview mode is used for some tests
dogtail tests: Allow import_media to handle empty search results better
dogtail tests: Make the "project" test count iconview items properly
dogtail tests: Initial cleanup and refactoring of the "project" test
dogtail tests: Refactor the project settings test and correct wrong assumptions

As resolution changed, pixel aspect ratio was also meant to change at the end.
The test doesn't pass, but that's because of bug #706544.
dogtail tests: Refactor the openProject helper to handle unsaved changes better

This is in preparation for the fix for bug #706539
mainwindow: Check for unsaved changes before (not after) loading a new project

This makes for a much less awkward flow and actually allows users to cancel
the closure of their current project if they are unsure.

Fixes bug #706539
medialibrary: Show the initial infobar again when creating a new project
dogtail tests: Refactor, fix and document the project loading and saving tests
application: Use quote_uri to make things simpler and a bit more robust
project: Remove some nonsensical code from loadProject

- Useless code since it wouldn't ever be run after return statements
- Empty return statements that are not actually needed as they are at the end
mainwindow: Pass exception messages correctly when errors occur while saving

This was a regression introduced by commit 267dd141
dogtail tests: Verify more carefully that backups load correctly/completely

Also do general refactoring to make the code more robust and easier to follow
project: Refactor and fix saveProject

- Don't use a "project" argument that is the same as self.current;
  this complexified the code and could be error-prone.
- Update self.current.uri only at the end of a successful save.
- Prevent a case where backups would hit an uninitialized "save" variable.
project: Remove useless conditions from saveProject

The first one is impossible as it is handled by the if/elif/else clauses above
The second one is redundant, as exceptions from GES are already caught earlier
project: Don't mangle the exceptions raised by GES while saving

This string had an incorrect syntax (should have been '_("foo") % e'),
which made mainwindow's save-project-failed handler choke in some cases
(such as overwriting a project that has been marked read-only).

However, there's no point in saying "Cannot save with this file format" still,
as the error dialog has a header that says 'Unable to save project "%s"' and
the error strings raised by GES should be sufficient to use as secondary text.
project: Restore the project's URI after exporting a copy as a tarball