DOSBox GUI update

A status update on the DOSBox GUI project that I've adopted. The changes are small, but numerous.

The following should be fixed now:

• after using the keymapper, activated from the UI, the alt key is no longer stuck after closing the UI
• leaving the keymapper, activated from the UI, and then closing the UI no longer leaves window chrome unresponsive
• crash on S3 cards when leaving the keymapper, activated from the UI
• no cursor in full-screen mode

The first two bugs were actually caused by issues in the keymapper and its (de)activation. The third is an SDL issue caused by DirectDraw and the S3 driver. Full-screen mode may still be very flaky, but SDL's standard directdraw video mode simply ignores the mouse cursor leaving it up to the client to draw it.

New features and other changes:

• combobox selection where appropriate
• main configuration menu can be closed by pressing Escape
• message boxes with help text can be closed by pressing Enter or Escape
• fading in/out has been disabled

Known issues in this version that are on my list to fix:

• clicking outside a menu or combobox (or any popup window) inside the parent's client area, but not on any client controls does not dismiss the popup window
• crash when activating menu in full-screen mode when using OpenGL (observed on nvidia drivers -- exclusive mode cannot be set)
• performance may be very much sub par, possibly due to the way the screen is drawn
• some properties cannot be edited because they have compound values -- these controls will show as disabled (for example: sdl::priority, cpu::cycles)
• none of the "help" buttons show anything useful
• help windows may be too large for the screen or even if they fit, placed too low
• the "execute now" button in the autoexec editor leaves a spurious prompt