GUI: Add "debuglevel" command to Debugger base class.

This allows the debug level to be changed at runtime from the debug
console.
AGOS: Remove redundant "level" command from debugger.

The base class "debuglevel" command now provides the same functionality.
PARALLACTION: Remove leftover "debugLevel" command prototype.
AGOS: Switch opcode debugging to a debug flag, rather than level 2 hack.

This is now set by --debugflags=opcode rather than -d 2, though it will
still require a debug level greater than 0.
AGOS: Switch VGA opcode debugging to debug flag, rather than level 3.

This is now set by --debugflags=vga_opcode rather than -d 3, though it
will still require a debug level greater than 0.
AGOS: Remove leftover unused _debugMode variable.
AGOS: Switch script debugging to debug flag, rather than level 4.

This is now set by --debugflags=script rather than -d 4, though
it will still require a debug level greater than 0.
AGOS: Change "script" debugflag to "subroutine" as more accurate.

This flag is used to enable dumping of subroutine scripts at start.
AGOS: Switch VGA script debug output to debug flag, rather than level 5.

This is now set by --debugflags=vga_script rather than -d 5, though
it will still require a debug level greater than 0.
AGOS: Add image dumping to file enable by debugflag.

This previously required a code change and recompile to enable.
It can now be enabled or disabled at runtime using the "image_dump"
debug flag.
GUI: Minor further fixes to "debuglevel" command in Debugger base class.
SCUMM: Remove "level" command from debugger. Replaced by "debuglevel".

This required a small amount of extra code changes to ensure that
_debugMode is kept in sync when the debugger is used to change the
level.
GUI: Clarify "debuglevel" command output in Debugger base class.

This should make it clear that -1 is used for disable.
GUI: Add usage for "debuglevel" command output in Debugger base class.
AGOS: Disable image_dump debug command.
Merge pull request #462 from digitall/debugConsoleChangeLevel

Add command to change debug level to the Debugger base class.