fix(docx): slow table parsing (#2553)

* chore(docx): remove unnecessary import

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* fix(docx): simplify parsing of simple tables

Simplify the parsing of tables with just text (no rich cells).
Move nested function group_cell_elements out of _handle_tables for readability.

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* chore(docx): reuse method for finding inline pictures

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* chore(docx): format strikethrough text

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* tests(docx): use fixtures to avoid converting same file multiple times

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* fix(docx): remove unnecessary argument docx_obj in functions

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* tests(docx): add test for rich table cells

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* chore(docx): small improvements in backend and its unit tests

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

* chore(docx): parse superscript and subscript formatted text

Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>

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Signed-off-by: Cesar Berrospi Ramis <ceb@zurich.ibm.com>