Fix #584 Solve the Unicode error in configtool

Error appeared when using a language that uses Unicode characters.
Fix typos and improve help for reset option via CLI

[skip ci]
ConfigTool General tab update

Improved the overall layout of the General tab.
Added "Reset Settings" and "Help us translate"
buttons. Some refactoring to remove unnecessary
instance variables.
Squashed 'docs/source/_themes/' content from commit 21e875d

git-subtree-dir: docs/source/_themes
git-subtree-split: 21e875d3a53ce897089ad690d897252f6063349d
Merge commit '94c258077e4b0870baa1ffd3350cedd3d1dd7159' as 'docs/source/_themes'
Fix #571: Footer broken on RTD site
Remove stuff that's not needed for the theme
Update copyright

The copyright in the build configuration file shows in the default
footer.
Hide default footer in docs by hiding default copyright

The default footer only consists of the copyright statement, so we
can get rid of the footer by not showing the copyright.

Instead we show a custom footer thanks to @umcsdon.
Fix #588 Plugins use Config framework.

Each plugin class that saves configuration
settings to the plugin.conf file now has its own
CONFIG_CLASS that extends the Config class
from the freeseer.framework.config.core package

Plugin options are loaded and saved from/to file
via the plugin's Config class.

Refactored plugins:
- Jackaudio
- PulseSrcAudio
- Multiaudio
- DesktopLinuxSrc
- Firewiresrc
- Usbsrc
- Videotestsrc
- Picture-in-Picture (pip)
- Videopassthrough
- Audiofeedback
- Ogg Icecast
- Ogg output
- Videopreview
- RTMP Stream