gdb/tui: use correct setting to control asm window styling

gdb/tui: use correct setting to control asm window styling

Currently the TUI's asm window uses `source_styling` to control
styling.  This setting is supposed to be for styling of source files
though, and the asm window displays disassembler output.

We have a different setting for this `disassemble_styling`, which is
controlled with 'set style disassembler enabled on|off'.

Switch to use the correct control variable.

I've written a new test for this, but this required some additions to
the tuiterm library in order to grab character attributes for a screen
region.

Approved-By: Tom Tromey <tom@tromey.com>