Fix T71554: 'Hide Unselected' not working for certain selections

rBc6cbcf83d015 caused to early out e.g when not all faces were selected
(but surrounding faces were, so implicitly all vertices were selected).
Now take (mixed also) selection mode into account.

Maniphest Tasks: T71554

Differential Revision: https://developer.blender.org/D6254

Fix T68191: Make-instances-real doesn't select the new instances

2.79 also did this [select the new instances] which was useful.
2.79 also kept the instancer selected [this patch deselects]

Reviewed By: mont29

Maniphest Tasks: T68191

Differential Revision: https://developer.blender.org/D6233

Fix T55632: USE_DRAG_MULTINUM and USE_ALLSELECT not working well together

'is_copy' was not set correctly on all uiButMultiState (it was done once
for uiHandleButtonData), resulting in 'delta' being used on some indices
of the array and not others in `ui_selectcontext_apply`.

Maniphest Tasks: T55632

Differential Revision: https://developer.blender.org/D6201

Fix T71372: view layer is not maintained when area is duplicated into new window

Maniphest Tasks: T71372

Differential Revision: https://developer.blender.org/D6205

Fix T71405: Trying to Ctrl-C on many buttons crashes Blender

Usually Ctrl+C copies the operator name to the clipboard
["bpy.ops.material.new()", "bpy.ops.object.material_slot_remove()"]
Crash happens for all buttons of UI_BTYPE_BUT without associated
operator [some are defined with callbacks only, often these are created
with e.g uiDefIconBut (instead of e.g. uiDefIconButO)]

Other examples that crash with Ctrl+C:
- animation decorators next to animatable properties
- button to show a modifier texture in the texture tab
- ...

2.79 survived here (result in the clipboard was just not changed hitting
Ctrl+C on these buttons), this is what happens with this patch as well.

Maniphest Tasks: T71405

Differential Revision: https://developer.blender.org/D6208

Revert "Fix T71126: add old hotkeys to set start/end frame in the timeline to"

This caused T71483.
Having timeline and dopesheet merged in 2.8 makes using S/E keys in timeline not work well.
2.7x keymap needs to fit with 2.8x internals, so probably 2.7x keymap will have to live with Ctrl+Home / Ctrl+End here.

This reverts commit 4fbcbbfb967cb0fdff11dfc1646a1c55bca96f1d.

Merge branch 'blender-v2.81-release'

Fix T71508: wrong gravity settings in scene defaults

Typo in rBf5e0dfe59c7e.
Showed when creating a new default scene.

Maniphest Tasks: T71508

Differential Revision: https://developer.blender.org/D6229

Merge branch 'blender-v2.81-release'

Fix T71452: instance collection to scene from outliner not incrementing collection usercount

Maniphest Tasks: T71452

Differential Revision: https://developer.blender.org/D6222