MailFolderCache: Avoid g_signal_handlers_disconnect_matched().

store_info_unref() already disconnects the signal handlers one by one.
folder_tree_model_folder_renamed_cb: Avoid duplicate rows.

Testing IMAP NOTIFY and folder renames, I observed a "folder-created"
signal emission before the "folder-renamed" signal emission.  So the
renamed folder already exists in the tree model when we receive the
"folder-renamed" signal.  Make sure we handle that sanely.
Updated Slovenian translation
Revert "Updated German help translation"

This reverts commit 83ce49534bc02a574b77a4072fdb973f43db8acf.

This commit introduced a build break:

    Error: Could not merge translations:
    'NoneType' object has no attribute 'node'
    make: *** [de/de.stamp] Error 1
Update API documentation.
NEWS update for 3.9.90 release.
Post-release version bump.
Updated Polish translation