xlsx parser does not handle inline strings.

If I have understand correctly ECMA-376 does define possibility to strings been inline rather in sharedStrings.xml.

From ECMA-376 Part 1 these should look like:

<c r="A1" t="inlineStr">
    <is><t>This is inline string example</t></is>
</c>

Orcus's xlsx parser does not handle this case even though there is some foundations for it (xlsx_sheet_context::m_cur_cell_type seems to be correctly set).

I can implment this I just want to make sure my aproach is valid:

1. In xlsx_sheet_context::start_element_cell handle XML_t and set this->m_cur_value.
2. In xlsx_sheet_context::push_raw_cell_value() handle xlsx_ct_inline_string by pushing to this->m_pool before calling this->m_sheet.set_string or somehow call iface::import_shared_strings::append_segment and iface::import_shared_strings::commit_segments.