Retext

* ReText now tries to load the icon theme from system settings if
Qt cannot auto-detect it.
* Added a GUI option to change the editor font.
* Added appdata file for appstream.

* Editor now displays cursor position in bottom-right corner.
* Added FakeVim mode (contributed by Lukas Holecek).
* ReText now shows a notification when the file was modified by another
application, to prevent data loss.
* WebPages generator removed (as better alternatives exist).
* Plain text mode removed.
* Added ability to configure file extensions for Markdown and
reStructuredText.

* Table editing mode.
* New settings: `colorSchemeFile` and `uiLanguage`.
* More settings are now configurable from GUI.
* Code base simplification and modernization.
* Dropped support for Qt 4.
* Added testsuite.

* Added configuration dialog.
* Added current line highlighting and line numbers support.
* Added support for PyQt5 and PySide libraries.
* Use new signals/slots syntax.
* Added option to select file encoding.
* Dropped support for Python 2 and support for running without WebKit
installed.

* Switch to pymarkups backend.
* Switch to Python 3 by default.
* Split `retext.py` to smaller files.
* MathJax support.
* Tab now inserts 4 spaces by default.
* Automatic indentation of new lines.
* External links are now opened in a web browser by default.
* Support for per-document CSS stylesheets.

* Spell checker suggestions.
* Markup-specific highlighting.
* Re-written parser and document-type logic.
* Lots of code clean-up.