There were three distinct mechanisms for signaling breakpoint changes in DolphinQt, and the wiring had room for improvement. The behavior of these signals has been consolidated into the new `Host::PPCBreakpointsChanged` signal, which can be emitted from anywhere in DolphinQt to properly update breakpoints everywhere in DolphinQt.

This improves a few things:
- For the `CodeViewWidget` and `MemoryViewWidget`, signals no longer need to propagate through the `CodeWidget` and `MemoryWidget` (respectively) to reach their destination (incoming or outgoing).
- For the `BreakpointWidget`, by self-triggering from its own signal, it no longer must manually call `Update()` after all of the emission sites.
- For the `BranchWatchDialog`, it now has one less thing it must go through the `CodeWidget` for, which is a plus.

Allows for avoiding string copies. While we're at it, we can also mark
ConvertTextToBytes as const.

This fixes a problem I was having where using frame advance with the
debugger open would frequently cause panic alerts about invalid addresses
due to the CPU thread changing MSR.DR while the host thread was trying
to access memory.

To aid in tracking down all the places where we weren't properly locking
the CPU, I've created a new type (in Core.h) that you have to pass as a
reference or pointer to functions that require running as the CPU thread.

Don't re-create the table on every update.