metadata-manager: Remove singleton

GeditDocument may attempt to use GeditMetadataManager after GeditApplication has
already called gedit_data_manager_shutdown. This happens because right now
_gedit_tab_load is not cancelled when the tab is closed, but it's only cancelled
if the user dismisses the loading operation from the infobar.

Also cancelling _gedit_tab_load is not enough because GtkSourceFileLoader
increases the reference count of the source buffer (in the case the
GeditDocument) until the async operation is completed or cancelled, hence
GeditDocument will attempt to use GeditMetadataManager in gedit_document_dispose
after GeditApplication has already called gedit_data_manager_shutdown.

To properly solve this - and avoid similar issues - make GeditMetadataManager
reference countable and keep an hard reference in each GeditDocument and in the
GeditApp.

Closes: #145
1 job for master in 6 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #274599
flatpak

00:06:49