This patch contains four commits to be individually:

• Cleanup: Rename USER_APP_LOCK_UI_LAYOUT
• Rename show_layout_ui > show_corner_split and remove from UI
• App Settings: Regions Visibility Toggle
• App Settings: Edge Resize

Reasoning for the rename:

There will be other settings that lock other aspects of the
UI layout (e.g., resizing of editors). So better to name
this setting what it actually handles (the corners).

New name: USER_APP_LOCK_CORNER_SPLIT

Note that this breaks API compatibility. However we are now grouping this
setting in the proper section (preferences.apps), so scripts had to update
anyways. So they may as well do it for the final name.

As for the User Interface:

The reason to remove from the UI is that this is intended for app setup,
and as such it should not be exposed to final users until we have apps
better presented (for 3.1 hopefully).

I have a separate patch that expose those settings to the UI in a Apps tab.
HoweverI would rather we do it properly later (3.1?) once the design for
app templates is better defined.