Write a fairly simple C++ widget qml plugin tutorial

The tutorial is based on the mediaframe widget. The metadata.json has a fair bit left out to simplify things. (I left out the Icon etc)

• The translate C++ strings section need more work.
• Add a few headings to breakup the huge new section.
• Add download link, install/uninstall and testing instructions.

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Random Plasma Widget:

• Fix missed filepaths in Examples page
• Use "Since:" instead of "Note: Since:"
• Fix snippet macro typo in widget setup page
• Better example and add missed filepaths in i18n page. Also replace more 0xa0 with spaces in directory trees.

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• Fix updateTOC() with really large offsets between headings
  Previously the TOC would highlight the next section even when it's not visible. It now calculates how much screen height is left in the current section. If less than 30% of the screen is the previous section, then the next section is being read.
• Rename code-filepath.html snippet to plasma-doc-style.html and add a few styles.
  • Remove top-margin from .highlight:first-child in .twocolsection.
  • Don't use 80% width for content in <sections> snippet. Text was wrapping too soon.
  • Add spacing and <hr> like line separator between .twocolrows.