PowerPC: Add functions to read/write the full timebase value

These can be moved into the RegisterColumn constructor, which avoids
potential allocations in the case a std::function would otherwise need
to allocate to hold all of it's captured data.

Also tidy up the inclusion order while we're at it.

Allows us to get rid of a silly pointer cast and deduplicate some code
from the front-end when it comes to reading the value.

Rather than introduce this handling in every system instruction that modifies
the FPSCR directly, we can instead just handle it within the data structure
instead, which avoids duplicating mask handling across instructions.

This also allows handling proper masking from the debugger register
windows themselves without duplicating masking behavior there either.

Qt/MemoryWidget: Allow operating on columns

Moves more common code into the Common namespace where it belongs.