Created global tag <release-19.11>. maintenance

Merged with default branch to prepare a new release. maintenance release-19.11

MicroPython: corrected the time synchronisation for the PyBoard.

Updated Russian translations.

EditorFilePage: added a button to install the PyMdown extensions.

Editor, Previewer:

Error Message Dialog:

eric6_browser: moved the scheme registration to the main file to fulfill a Qt requirement.

eric6: removed some 'experimental' code.

PreviewerHTML, ExporterHTML:

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eric ide