Initial framework and basic reading of file-lists

Initial steps for T88184.

Adds a `filelist/` directory to `editors/`, containing C++ classes for
(recursively) reading directories and files into a file-list. The logic is
based on the File-Browser file-list, so that the actually executed logic is
basically the same (avoiding regressions or old issues re-appearing). But it
is entirely re-designed to work for both the asset system and the file browser.

For more info about the design, see T88184.

Also contains "peeking" the file list to count the files to be read. This is to
improve progress reporting later. However, it has some issues and I'm not sure
actually it's something we'll even need in the end, I see other ways to improve
things here.

Fix warning on Apple Clang, missing override specifier

Was giving a warning:
```
BKE_spline.hh:293:35: warning: 'interpolate_to_evaluated_points' overrides a
member function but is not marked 'override' [-Winconsistent-missing-override]
```

Snap based on the bounding box when dropping, not origin

This makes the dropping use the object's bounding box for snapping, so
that it matches the bounding box we draw while dragging.

Not that this may have to be optional in future, there may be valid
cases to use the origin instead of the bounding box for snapping.

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging

Fix T82002: ENTER does nothing when mouse cursor is over save dialog text field

The `file.execute` operator would early-exit because the mouse wasn't hovering
the file list. Caused by 4ba9d7d71e8f.

Although simpler solutions would have been possible, I decided it's better to
split add a new operator for executing based on the mouse (for double-clicking
files), to separate that from the window level execute operator
(`file.execute`). This allows more control and we can get rid of the implicit
assumption that the keymap would call `file.select` on mouse-press, and
`file.execute` on double-click, for the double-click behavior to work. The cost
is that we execute the file selection & activation logic twice on the
double-click, but that shouldn't be an issue at all.
Also removes the `need_active` property from the `file.execute` operator.
That's ancient and wasn't implemented well anyway.

To be clear, reason this fixes the bug is that `file.execute` works entirely
with the `execute()` callback now and doesn't early-exit based on the mouse
position anymore.

Might trigger warnings about the `need_active` property not being found for
custom keymaps. These can be ignored and the property can safely be removed
from the keymap. I don't expect other keymap breakages.

Fix missing header & footer toggle chevrons in Spreadsheet

The little chevron tab to open a hidden region wouldn't show up in the
Spreadsheet editor. Cause was an incorrect GPU-scissor usage:
While drawing regions, the scissors should be kept enabled, just the
scissor rectangle should be updated - and afterwards reset to what it
was before.

Fix missing header & footer toggle chevrons in Spreadsheet

The little chevron tab to open a hidden region wouldn't show up in the
Spreadsheet editor. Cause was an incorrect GPU-scissor usage:
While drawing regions, the scissors should be kept enabled, just the
scissor rectangle should be updated - and afterwards reset to what it
was before.

Fix missing background of buttons after reports

Reports would change the active emboss of the Block, to make some of its
buttons appear without background. But it didn't reset it so that
buttons added afterwards wouldn't use the correct embossing.

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging