Dopesheet-Timeline: First steps towards making the Timeline a mode of the DopeSheet Editor

For many years, animators have been requesting the ability to edit keyframes in the
timeline. However, implementing such tools in the timeline quickly becomes a slippery
slope, where we'll eventually end up having to duplicate all the functionality from the
dopesheet editor.

Discussing with William and Pablo this morning, we realised that perhaps it might be possible
to just make the Timeline a mode of the Dopesheet Editor (and kill off the old standalone
Timeline), meaning that we essentially get all the Dopesheet Editor goodness for free!
Also, with some proposed UI updates (i.e. allowing "submodes" of editors to be part of the
the main editors selector), it might not even matter that there isn't an "actual" timeline
editor anymore.

This commit implements the following changes (which are actually sufficient for supporting
most basic workflows):
* Timeline mode in Dopesheet Editor
* Tweaks to UI code to make the Timeline header/menus show up in Dopesheet editor

TODO:
* Hide channels list when switching to timeline mode
* Port over cache-file indicators
* Add missing timeline-only settings that need a new home in the dopesheet
* Go through fixing all timeline editor operators (e.g. Bind to camera)
* Port over start/end frame shading (and adjust preview range rendering to make the
  distinction between these clear)
* Remove old timeline editor, and transfer over any leftover code

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