[DiskProbe] Simplify Jumping to File and Block Offsets (easy)

[bonefish]

My most frequent use case for DiskProbe is that I want to have a look at data at a specific file offset. The current interface requires me to calculate block number and block offset manually, enter the block number in the block text field, and then find the offset in the block. The support for expressions in the block text field helps a bit, since it allows to at least jump to the right block without need for manual calculation (e.g. "fileOffset / 512"), but it's still annoying enough to have to calculate the block offset.

Long story short, make the block offset and file offset fields editable too (ideally with expression support). And select the byte at the entered offset.

[TwoFx]

I worked on this for GCI (I didn't claim ​the task yet because I am waiting for something else to get reviewed).

[mmu_man]

Layout could probably be improved but it seems to work fine. Pushed as hrev48597.