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>

BDeskbar: Moved constants to shared header

* Instead of duplicating them in different files.