Multi-platform building editor (includes Linux)

Changes:

* Game data files (*.001, *.002, etc.) now build from editor rather than native (scripts are still compiled by native)
* Windows game files now build to Compiled/Windows.
* Raw game data files now build to Compiled root directory.
* Added AGS.Types.IBuildTarget interface to allow common interface between platforms and allow custom build targets.
* Build targets must derive IBuildTarget interface and register with AGS.Types.BuildTargetsInfo class or they will not be recognized, but may be registered by editor or plugins.
* Added BuildTargetDataFile, BuildTargetWindows, and BuildTargetLinux build target implementations.
* Compilation no longer crashes if acwin.exe is missing, though Windows build target becomes unavailable.
* Added building for Linux (requires "Linux" subdirectory in editor directory with Linux engines and libraries, or else Linux build target becomes unavailable).
* Provided way for editor to retrieve constants that need to be shared between engine and editor source (NativeConstants class).
* Replaced FlagsUIEditor and FlagsUIEditorControl with StringListUIEditor and StringListUIEditorControl, for more dynamic drop-down lists in Settings pane without relying on static enums.