DOSBox GUI update

Yet another update for the DOSBox GUI. Lots of small changes, no real biggies.

Changes:

• Activation of the keymapper is now deferred when in -startui mode
• Active page settings are properly stored when saving config file
• Added a mention of loadfix
• Changed the colours in the messageboxes to be hopefully a bit easier on the eyes
• Checkboxes now have focus rectangles
• Configuration save dialog box is now populated with default config file location
• Fixed some focus issues
• Properly synchronisation when using the advanced editors
• Removed the help buttons on the advanced editors, as there was no text for them anyway
• Restored the "Append History" button functionality in the autoexec editor
• Switched OK and Cancel buttons so OK is on the left, as is the norm for Windows dialog boxes
• Welcome message when opening the GUI, along with override checkbox

Remaining known issues:

• Crash when activating menu in full-screen mode when using OpenGL on nvidia graphics cards (exclusive mode cannot be set) -- this appears to be an Nvidia+SDL+OpenGL issue
• 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
• The help text for "speed" is overlong and runs off the bottom of the window.

• Add a GUI for mountings
• Bring the keymapper into this GUI format
• Remove the help messages or at least replace them with separate ones that are more appropriate for use inside the GUI
• Limit the input for edit controls to numbers where appropriate