Fix T62057: Particle settings keyframes not showing in Graph Editor channels

In short, the settings to expand/collapse the Particles Animation Dopesheet expander
were no longer getting exposed, so the F-Curves attached to the particle settings
were not showing up in the channels list as that section was collapsed and couldn't
be opened from the UI.

Early on during the development of 2.8, we originally wanted to completely remove
the Particle System. Eventually that decision got walked back, and so particles
were reinstated. Well... most of the relevant code was! One of the areas that was
the most messed up during this process was the animation editor support for these
channels. It seems that there was almost a two-step removal process here -
the first pass tried to keep the channel definitions while removing all references
to particle stuff, while the second pass tried to remove the definitions completely
and/or re-added them in the wrong places, etc. To say the removal/reverting history is
here is "colourful" is an understatement...

Cleanup: Adding braces around all cases here to make it easier for the next step

Fix: Better fix for KeyingSet "Export to File" and unescaped characters

Use Python's native raw formatting instead, which should take care of any issues
we may face.

Fix: Ensure that KeyingSet.bl_description field is properly escaped when using the Export to File operator

Previously, if double-quotes appeared in the KeyingSet.bl_description field,
these would cause a syntax error in the resulting .py script export of the
KeyingSet. Since single quotes are even more likely to appear
(e.g. as apostrophes), we now use triple quotes here.

Unreported bug, noticed earlier when investigating T61010.

Fix T60753: Graph Editor showing cache curves regardless of filters

This commit adds a datablock filtering option for cache files channels,
so that a shot with lots of these in addition to standard animation
(e.g. the Spring production files) don't become bogged down by these.

Furthermore, these channels also respect the "Only Selected" toggle too now.

Fix typos

Fix T59984: Drivers don't update values in UI or show proper errors

We now perform COW -> original data flushing for all the debug values + error
status flags on Drivers/DriverVariables/DriverTargets, as these are only set
when errors are encountered when evaluating drivers.

Typo fixes

Fix: Drivers popover not showing

This was broken since cca87ccc753a85b447872a259c9d3e3f07ec1c13

The problem was that as this popover was defined in the C-code
for the Graph Editor, the relevant panel never got added to the
WM_paneltype registry, since only Python defined panels get
WM_paneltype_add() called when they are defined.

Since the majority of panels defined for regions are only used there,
a more localised fix was applied here by only adding the driver popover
to the global list manually.

Fix typo