ZPixel portable mode

While this file sits next to photocraft.exe, ZPixel keeps its preferences, brush presets,
crash-recovery autosaves and startup state in the PhotoCraftData folder beside it, and writes
nothing to %APPDATA%. Copy the whole folder to move ZPixel with its settings.

Delete this file to use the normal per-user settings folder (%APPDATA%\Photocraft) instead.
If PhotoCraftData can't be written (for example on a read-only drive), ZPixel says so and
uses %APPDATA%\Photocraft instead.
