Pympress Versions Save

A handful of new features, mainly centered around configuration files and video support:

• Portable configurations, supported by ticking Starting Configuration > Portable installation
  This requires write permissions in the root install directory of pympress.
• All keyboard shortcuts are configurable in the config file.
• Support for Gstreamer as a video backend rather than VLC
• Configuration file locations now respect platform-specific conventions better
• Log file locations also respect platform-specific conventions and are written to user locations to avoid problems with multi-user systems
• Options for both video backends to enable/disable, set initialization options, and choose applicable mime types.
• Blank background colour can be black or white (white_blanking option under [content] in the configuration file)
• Configuration file pympress/share/defaults.conf containing default/fallback values.
  This file should not be modified by the user.

Includes a bug fix for python2, improving the log file mechanism and completing some translations.

Mainly a big refactor, some bug fixes linked to logging behaviour on Windows thanks to @r5r3