melonDS: dialogs

[msink]

fixes from melonDS related to dialog windows

[mischnic]

My old comment:

add support for non-resizable windows

#316 has macOS support (but as a setter and not a constructor argument)

[mischnic]

Example for filter from melonDS: "DS ROM (*.nds)|*.nds;*.srl|Any file|*.*"

Also, while we're at it: they also have initpath (though it isn't implemented):

_UI_EXTERN char *uiOpenFile(uiWindow *parent, char* filter, char* initpath);

[msink]

So should I drop first patch, assuming #316 is better and will be merged instead?