Issue/317 fixing block elements selection range

[khaykov]

This partially fixes #317, and closed as duplicate #380
"Horizontal Row" part of the #317 is not technically related (and looks like way more involved), so I'll address it later.

Test 1

1. Load demo app.
2. Place cursor in Heading 2
3. Open Heading menu and select Default
4. Open Heading menu and select Heading 2
5. Open Heading menu and confirm that Heading 2 is selected.

Test 2

1. Launch demonstration app.
2. Move cursor anywhere in "Bold" text.
3. Tap Ordered List or Unordered List format button.
4. Tap Heading format button.
5. Notice Default paragraph is selected.

[0nko]

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