Use non-const XNode in ActorUtil::LoadFromNode

No-op aside from constness.  Need this later.
Re-add FT_Xml and FT_Sprite to FileType enum

Doesn't associate any behavior with them yet.
Warn on failed command compilation
Foreground: don't propagate OnCommand

We load foreground changes at the beginning of the screen, but they
shouldn't be issued an OnCommand until it's time for them to start.  The
Foreground layer, however, should get the OnCommand right away.  So
don't propagate it, since we already issue On manually to changes.
Foreground: don't issue a spurious OnCommand

We don't need the ending beat until the change has already started, so
just use the legitimate On to calculate things rather than doing
anything hacky.
BitmapText: check both File and Font as path

Allow BitmapText to specify font from Theme/Fonts as well as locally,
using either the Font or the old-style File attribute.
Player: broadcast StepPN message (compat)

For backwards compatibility, broadcast the old StepP1/2 message in
addition to the new Step message.  New themes, charts, etc. should of
course use the new message with parameters.
Resurrect Annotate function for XNodes

No-op.  Function isn't actually used yet.
Command/LuaManager: add legacy command parsing

Only enabled when compiling via CompileXNodeTree.
Annotate XNode trees with legacy attribute
ActorUtil: add loading behavior for legacy actors

Filetypes are not yet linked in, so this shouldn't change anything.

The loading/guessing logic is ported from OpenITG.
Reconnect legacy sprites
Process FT_Xml as legacy actors

Everything from here on out is edge cases.
Merge pull request #54 from djpohly/xml-forwardport

Resurrect XML actor support when QuirksMode=1