cmd/syncthing, lib/api: Separate api/gui into own package (ref #4085) (#5529)

* cmd/syncthing, lib/gui: Separate gui into own package (ref #4085)

* fix tests

* Don't use main as interface name (make old go happy)

* gui->api

* don't leak state via locations and use in-tree config

* let api (un-)subscribe to config

* interface naming and exporting

* lib/ur

* fix tests and lib/foldersummary

* shorter URVersion and ur debug fix

* review

* model.JsonCompletion(FolderCompletion) -> FolderCompletion.Map()

* rename debug facility https -> api

* folder summaries in model

* disassociate unrelated constants

* fix merge fail

* missing id assignement

lib/ur/memsize_windows.go

@@ -0,0 +1,30 @@
+// Copyright (C) 2014 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package ur
+
+import (
+	"encoding/binary"
+	"syscall"
+	"unsafe"
+)
+
+var (
+	kernel32, _             = syscall.LoadLibrary("kernel32.dll")
+	globalMemoryStatusEx, _ = syscall.GetProcAddress(kernel32, "GlobalMemoryStatusEx")
+)
+
+func memorySize() (int64, error) {
+	var memoryStatusEx [64]byte
+	binary.LittleEndian.PutUint32(memoryStatusEx[:], 64)
+
+	ret, _, callErr := syscall.Syscall(uintptr(globalMemoryStatusEx), 1, uintptr(unsafe.Pointer(&memoryStatusEx[0])), 0, 0)
+	if ret == 0 {
+		return 0, callErr
+	}
+
+	return int64(binary.LittleEndian.Uint64(memoryStatusEx[8:])), nil
+}