Update animation editor

This commit makes several changes:

-Remove console command "pac_in_editor" and use the net library to
transmit this information instead (this is used for the overhead "In
Editor" text).

-Replicate this behavior within the Animation Editor.

-Add a "Parsed Skeleton" option to the animation editor that allows
users to use any bone structure their current player entity has.

-Disallow creating an animation without a name, which would cause errors
later on.

-Don't display boneanimlib animations that were created by the
custom_animation pac part

-When editing a sequence, t-pose the player so the user has a proper
frame of reference

-Automatically save backups of the current animation when the editor is
closed

-Allow any user to use the animation editor

-Preserve frame length when cloning a keyframe

-Various bugfixes