fix strange behavior on Backspace press in ST

Before:
if text was selected and cursor was at indention, one indention (two
spaces) was removed.

if cursor was at start position of the line, then nothing happened.

if cursor was in the middle of indention then on backspace more
indention were added


Now:
if text is selected, it will be removed.
if cursor is at start position, then new line will be removed.
if cursor was in the middle of indention one indention is removed.

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Beremiz's user manual

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Contents:

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.. toctree::

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   :maxdepth: 2

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   install

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   start

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   edit

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   build

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   connectors

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   debug