Fix double free

Carl Schwanrequested to merge
work/carl/double-free into master

Instead of having the post use the QObject parent for memory management rely only on the post being inside std::shared_ptr

The only exception being the newPost method that create the post that the user is writting.

Alternative solution to !77 (merged)

@vkrause @redstrate

Edited by Carl Schwan

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