Debugger and UI cleanup

While the TIA work is basically complete, we're now working on porting all missing features from Stella 4.x and/or improving the code. For this pre-release we concentrate on debugger stuff:

• The TIA core has improved optimizations and caching, allowing for up to 100% performance improvements; that means that all ROMs should be tested again, to make sure no regressions were introduced
• TIA tab in the debugger now works correctly for all widgets which existed in 4.x
• Non-editable UI items in the debugger are now shown with a different colour
• The debugger 'cls' no longer erases history
• Added DelayQueueWidget to TIA tab
• Debugger trap/read/write now work like the 'm' versions from Stella 4.x
• Added 'palette' and 'debugcolors' prompt commands
• Fixes for compiling with gcc 7+, Visual Studio 2017 and OSX Sierra
• Updated BUS and CDF bankswitch schemes
• Added ability to see contents of 'old' player/missile/ball registers
• Added 'crossed out' graphics to certain debugger UI objects, to clearly show they're not currently active
• Fixed long-standing bug in DPC+ 3-voice music output
• Added 'fixed debug colours' info to the TIA tab, including ability to toggle it