display: Allow forcing viewport scrolling when moving units

This allows forcing the viewport to scroll when moving units even when
[lock_view] is in effect or Follow Unit Actions is disabled in Advanced
Preferences by passing an optional parameter to move_unit() and the
unit_mover class constructor.
Add [move_unit_fake] force_scroll attribute, defaults to 'yes'

This causes the viewport to be scrolled during MUFs even when
[lock_view] is in effect or Follow Unit Actions is disabled in Advanced
Preferences.

Rationale for the default value:
 * Follow Unit Actions is already ignored by [scroll]/[scroll_to] and
   unit [message]s, even in 1.10.x.
 * [move_unit_fake] may be used in cutscenes in conjunction with
   [lock_view]. It is desired to have the viewport scroll to show
   unit movement, and this isn't possible at the moment without
   overriding the implementation of [move_unit_fake] using Lua to
   disable the viewport lock for the duration of the action.
 * Since the above is a rather common scenartio, it is better to
   let the engine handle it correctly without additional code in
   the WML/Lua side. For situations where the [move_unit_fake]
   scrolling is not desired, the user can pass the optional attribute
   themselves.
 * Follow Unit Actions's usefulness outside of unscripted battle
   maps is rather questionable.
Add [move_unit] force_scroll attribute

Defaults to using the [move_unit_fake] default.