fix bug: branching at a node with null swipe_id created broken chat

[Technologicat]

As it says on the tin.

I don't know how I didn't catch this before - maybe it's only some chat files where a message has a null swipe_id.

Anyway, fixed, works now.

EDIT: Sneaked in another small bugfix.

Changelog:

• Fix the branch button in the full info panel sometimes creating a broken chat
• Fix exception in Timelines when hitting Esc to close the Manage chat files view (of core ST)
• Fix and improve consistentency in handling of zoom to fit (no more uncaught exceptions from this, hopefully)
• UX: Show loading spinner while the timeline view is opening
• UX: Fix incorrect position of root node (especially if the character has lots of chats)

[Technologicat]

Argh, still finding more bugs. This time, there was an uncaught exception in the log when opening the timeline view and the text search was empty.

I'll let you know when there are no more bugs for now. Probably tomorrow.

[Technologicat]

I think that's all for now. This PR is complete.

[Technologicat]

Thanks for the merge!