Buttons/commands state is based only on start of the selection

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Similar to #525

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Justify button are disabled when disallowed element is selected. However, when allowed element is partially selected together with disallowed element, buttons are enabled. It only happens when selection starts in allowed element and ends in disallowed one:

For the reverse situation, buttons are disabled:

This is visible after implementing #455, however it might be also a case for other plugins. So basically, buttons/commands state should not be based on the beginning of the selection, but maybe it should be based on a whole selection (so such inconsistent situations as the one above does not take place).