TextElement get_text(): add tspan tail + recursion

What does the merge request do?

This MR improve the get_text() function from TextElement class:

• The tail of the tspan will be included in the output
• The text value of a tspan is resolved recursively

This is a solution to the problem of issue #549 and it allows the user to access all the tspans recursively if needed but not from a single call to tspans().

Summary for release notes

Allow to fetch all the text from a text element when using tspan

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history