Add love.window.showFileDialog(type, callback [, settings]).

Resolves #990.

The type parameter is an enum: "openfile", "openfolder", or "savefile".

The callback parameter takes 3 args:
  files (array of full platform-dependent paths to selected files, empty if the dialog is canceled by the user),
  filtername (nil if none is set, see below),
  errorstring (nil if no error).

The optional settings parameter of showFileDialog has the following fields:
{
  title = string (optional title),
  acceptlabel = string (optional custom name for the Accept button),
  cancellabel = string (optional custom name for the Cancel button),
  defaultname = string (optional default folder/file name for the file text box),
  filters = table (filter names as keys and filter pattern strings as values).
}

src/modules/window/wrap_Window.cpp

@@ -20,6 +20,7 @@
 
 #include "wrap_Window.h"
 #include "sdl/Window.h"
+#include "common/Reference.h"
 
 namespace love
 {
@@ -647,6 +648,110 @@ int w_showMessageBox(lua_State *L)
 	return 1;
 }
 
+static void fileDialogCallback(void *context, const std::vector<std::string> &files, const char *filtername, const char *errstr)
+{
+	auto r = (Reference *)context;
+	lua_State *L = r->getPinnedL();
+
+	r->push(L);
+
+	lua_createtable(L, (int)files.size(), 0);
+	for (size_t i = 0; i < files.size(); i++)
+	{
+		lua_pushstring(L, files[i].c_str());
+		lua_rawseti(L, -2, (int)i + 1);
+	}
+
+	if (filtername != nullptr)
+		lua_pushstring(L, filtername);
+	else
+		lua_pushnil(L);
+
+	if (errstr != nullptr)
+		lua_pushstring(L, errstr);
+	else
+		lua_pushnil(L);
+
+	int err = lua_pcall(L, 3, 0, 0);
+
+	delete r;
+
+	// Unfortunately, this eats the stack trace, too bad.
+	if (err != 0)
+		throw love::Exception("Error in file dialog callback: %s", luax_tostring(L, -1));
+}
+
+int w_showFileDialog(lua_State *L)
+{
+	Window::FileDialogData data = {};
+
+	const char *typestr = luaL_checkstring(L, 1);
+	if (!Window::getConstant(typestr, data.type))
+		return luax_enumerror(L, "file dialog type", Window::getConstants(data.type), typestr);
+
+	luaL_checktype(L, 2, LUA_TFUNCTION);
+
+	if (!lua_isnoneornil(L, 3))
+	{
+		luaL_checktype(L, 3, LUA_TTABLE);
+
+		lua_getfield(L, 3, "title");
+		if (!lua_isnoneornil(L, -1))
+			data.title = luax_checkstring(L, -1);
+		lua_pop(L, 1);
+
+		lua_getfield(L, 3, "acceptlabel");
+		if (!lua_isnoneornil(L, -1))
+			data.acceptLabel = luax_checkstring(L, -1);
+		lua_pop(L, 1);
+
+		lua_getfield(L, 3, "cancellabel");
+		if (!lua_isnoneornil(L, -1))
+			data.cancelLabel = luax_checkstring(L, -1);
+		lua_pop(L, 1);
+
+		lua_getfield(L, 3, "defaultname");
+		if (!lua_isnoneornil(L, -1))
+			data.defaultName = luax_checkstring(L, -1);
+		lua_pop(L, 1);
+
+		lua_getfield(L, 3, "filters");
+		if (!lua_isnoneornil(L, -1))
+		{
+			luaL_checktype(L, -1, LUA_TTABLE);
+
+			lua_pushnil(L);
+			while (lua_next(L, -2))
+			{
+				Window::FileDialogFilter filter = {};
+				filter.name = luax_checkstring(L, -2);
+				filter.pattern = luax_checkstring(L, -1);
+				data.filters.push_back(filter);
+				lua_pop(L, 1);
+			}
+		}
+		lua_pop(L, 1);
+
+		lua_getfield(L, 3, "multiselect");
+		if (!lua_isnoneornil(L, -1))
+			data.multiSelect = luax_checkboolean(L, -1);
+		lua_pop(L, 1);
+
+		lua_getfield(L, 3, "attachtowindow");
+		if (!lua_isnoneornil(L, -1))
+			data.attachToWindow = luax_checkboolean(L, -1);
+		lua_pop(L, 1);
+	}
+
+	// Save the callback function as a Reference.
+	lua_pushvalue(L, 2);
+	Reference *r = new Reference(L);
+	lua_pop(L, 1);
+
+	instance()->showFileDialog(data, fileDialogCallback, r);
+	return 0;
+}
+
 int w_requestAttention(lua_State *L)
 {
 	bool continuous = luax_optboolean(L, 1, false);
@@ -701,6 +806,7 @@ static const luaL_Reg functions[] =
 	{ "isMaximized", w_isMaximized },
 	{ "isMinimized", w_isMinimized },
 	{ "showMessageBox", w_showMessageBox },
+	{ "showFileDialog", w_showFileDialog },
 	{ "requestAttention", w_requestAttention },
 	{ "getPointer", w_getPointer },
 	{ 0, 0 }