Add in the ability to add a cart save as a gameshark save to an N64 mempack file

[euan-forrester]

We can currently extract these, but we can't take a cart save and convert it and/or add it to a mempack file.

Might be easier to convert it to its own mempack file rather than having the ability to add it. How would be differentiate which are regular saves and which are cart saves? Need to think here.

We'd need to truncate any empty pages from the end of the cart save

This would let people use their gameshark to write saves downloaded from the internet to their carts

[euan-forrester]

Should be really easy: just change N64MempackSaveData.createFromSaveFiles() to check for the gameshark publisher etc code and trim the save buffer. Everything else is done (see N64MempackSaveData.parseFilename(), and where it's called)

[euan-forrester]

Also add in a help icon explaining how files are chosen to be either regular mempack saves or cartridge saves (to both dexdrive and mempack screens)