Fix URLs in the development docs

* Fix dead links where possible
* Use online instead of local links to the BeBook

Change-Id: I250117dcccc0026925c56545cca7e3b4467f2c78
Reviewed-on: https://review.haiku-os.org/c/811
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

Deskbar: Resizable tray

* Adds max width and height arguments to
instantiate_deskbar_(item|entry).
* Old applications just stay with a 16x16 scaled icon, though.
* All used apps within the repository are converted to the new call
besides the input_server input method icon (that will need further
API changes in the input_server).

Change-Id: I29cc439396917be2c24135888459d31364997dff
Reviewed-on: https://review.haiku-os.org/656
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/develop: Mass directory restructure.

Now vaguely follows the tree structure of "src", with the exception of
directories that described subsystems spanning more than one "kit" or
"server" (e.g. "media", "midi", "bluetooth") -- these have been left as their
own top-level directory within docs/develop.