textblock/optimization: refactor evas_textblock_style for memory and perfromance.

textblock/optimization: refactor evas_textblock_style for memory and perfromance.

the main user of textblock_style_set() api is the edje whcih keeps its owen edje_textblock_style
tags( string_shared string) by changing the textblock to keep the string_shared string will improve
the chance of sharing the same string hence reducing memory.

By removing the Eina_StrBuf usage inside the loop in textblock_style_set() api we can avoid lot
of temporary memory allocation and deallocation hence will improve performance.

Note: I see lot of places we use Eina_Strbuf inside a loop (eina_strbuf_new() does 2 allocation)
We need to be extra carefull while reviewing when the code uses those construct to see if its really necessary.

Data: it reduces memory allocation by 7000 in elementary_test launch time.

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D9545