Add BIconUtils::GetSystemIcon

Allow to easily access the alert icons and anything that's added to app_server
resources.

Fixes #10887.

Convert BAlert, Debugger AlertWithCheckbox and Keymap ModifierKeysWindow
to make use of it, removing the duplicate code to locate app_server
resources.

The resources are initialized only once (per application), so there is no need
to reload them for every access to the icons.

In the ticket there is discussion about putting this in BControlLook,
but I think this should in fact be moved fully into app_server with
special drawing commands for well-known icons. That would avoid loading
and rendering the icon on the application side to then send it to
app_server (especially in remote_app_server case)?

In any case, this simple API can serve as a base for applications to
use, and we can change how it is implemented later on.

Change-Id: Id370526ae5cf165cfb8bc277bc8a7f46c26f542d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6463
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Merge liblocale.so into libbe.so.

* Make the locale kit a part of libbe.
* Drop the LocaleBackend kludge used from within libbe (and from
other places, too) in order to access system catalog strings.
This is now done via gSystemCatalog, which is provided and initialized
by libbe.
* Drop all references to liblocale.so from all Jamfiles.
* Add legacy symlink liblocale.so in order to keep optional packages
that rely on it in a working state.

TODO: the documentation hasn't been updated.

Merge liblocale.so into libbe.so.

* Make the locale kit a part of libbe.
* Drop the LocaleBackend kludge used from within libbe (and from
other places, too) in order to access system catalog strings.
This is now done via gSystemCatalog, which is provided and initialized
by libbe.
* Drop all references to liblocale.so from all Jamfiles.
* Add legacy symlink liblocale.so in order to keep optional packages
that rely on it in a working state.

TODO: the documentation hasn't been updated.