Allow raw partitions

[GXTX]

Currently the project expects the image to be that of a full drive. Allow single partition mounting. This would allow for modifying Chihiro images, and possibly XMUs but I think they're slightly(?) different.

[aerosoul94]

Related: #7

This is actually something I thought about recently. One thing to note is that if the given size of the partition is wrong, it could make it fail to mount since the file allocation table size is calculated based on the size of the partition. You can actually manually add partitions right now using the Add Partition (Drive menu -> Add Partition) feature but that would only work if the drive format is correctly detected, so that's another issue I need to fix. I want to add a feature that would automatically find partitions and calculate their size but that will probably be in the future. I think what I can do now is fix drive detection, since it currently incorrectly defaults to detecting a retail xbox 360 drive, and then add an Endianess option to the Add Partition dialog window.