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>