Add ability to embed resources into .ptex files

[RobotRoss]

Feature/enhancement description: For those of us who use Material Maker across devices and operating systems, the current method of linking external resources (e.g. images) in .ptex files is to create a static link to the file. However, this approach breaks down very quickly if you need to then transfer the material into another Material Maker instance, e.g. on another computer or another operating system on the same computer.

Instead, it might be useful to give the user the option to embed all resources into the .ptex file itself, removing the need to link the files within the host operating system and increasing the portability of ptex files.

The tradeoff, is of course that the size of the ptex files themselves will inflate proportionately to the number (and size) of the embedded resources; however, to many users, this is a worthwile tradeoff.

[RodZill4]

If you put your images and .ptex file in the same directory, you just have to move all of them (the .ptex contains a relative path in that case). Putting images inside the .ptex file is not an option (because I don't want muti-megabytes non-procedural materials on the website, for obvious reasons). Maybe a tool to archive/unarchive materials?