Release branch for openshot-qt 2.5.1 (required libopenshot version 0.2.5)

Bump version to 2.5.1 (and update translations, cache, supporters)

Save repaired data with indents / pretty-printing

If image is not present in audio file the thumbnail is still needed,
otherwise it will be rendered as black rectangle in the Timeline.

Add pending_action tracking to update manager

Protect filter() against damaged projects

json_data: Autorepair corrupted project files

If a file is detected as having corrupted unicode escapes, we make
a backup copy as filename.osp.bak (or filename.osp.bak.1...), then
regexp-replace all of the broken escapes with corrected ones. Then
we run the file through json.loads() and json.dumps() again to remove
the escaped characters, and write the results back to the original
file location as UTF-8.

Make Unicode path for projects work again

It's been three years since OpenShot stopped inserting those envvars.
If anyone still has them set today, it's likely they were created by
some other software and we shouldn't be removing them.

Fixing logic when trying to parse changelogs in official releases

Docs: Outdent table code a bit

Automatically create and update sphinx documentation on openshot.org

Fixes an issue when read only fields of properties are hard to read on
bright backgrounds of the Table of Properties for the system theme (not
Dark one).

Fixes an issue that list view miniatures of the imported clips not
rendered if path contains non-English letters.

By defult the ".text" field in response uses ISO-8859-1 charset (RFC
2616 specs) while server sends utf-8 encoded path string just few
strings below in the code.

Fixes an issue when application fails to load files from the saved
project with non-English characters in the path to the clips.

Also removing json.loads strict=False key as obsolete in 3.x Python.

Upgrade the Travis YAML control file to:
- Use the 'language: python' test environment
- Install deps with the Travis apt plugin, instead of sudo
- Install cx_Freeze with pip3, rather than from a URL
- Add a Python 3.6 test build, running on Bionic

Tutorial: Mouse cursor, painting fixes; clean up widget objects

Remove outdated Doxyfile.in