Consume new entries, and add them to the model

LIU,JIAHUI requested to merge H00ly666/gnome-logs:work4_liujiahui into master

Receive the signal to consume the new entries, the approach to
adding them to the model seems similar to the fuction
gl_journal_model_fetch_idle (), so factor out the common code
to be a separate function gl_journal_model_handle_add_entry ().

The function gl_journal_model_add_header () was modified to be
reused for adding the new header.

Edited by LIU,JIAHUI

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