task-row: Reorganize header

This commit removes the GtkStack from GtdTaskRow, and exposes
the title entry (that is now a GtdExpandableEntry) directly
in the title.

This effectively implements the proposed behavior of #107
1 job for wip/gbsneto/row-header-improvements in 5 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1020
build-todo

00:05:28