gui, lib/config, lib/model: Allow absolute values for minimum disk free space (fixes #3307)

This deprecates the current minDiskFreePct setting and introduces
minDiskFree. The latter is, in it's serialized form, a string with a
unit. We accept percentages ("2.35%") and absolute values ("250 k", "12.5
Gi"). Common suffixes are understood. The config editor lets the user
enter the string, and validates it.

We still default to "1 %", but the user can change that to an absolute
value at will.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4087
LGTM: AudriusButkevicius, imsodin

cmd/syncthing/main.go

@@ -1117,7 +1117,7 @@ func defaultConfig(myName string) config.Configuration {
 		defaultFolder = config.NewFolderConfiguration("default", locations[locDefFolder])
 		defaultFolder.Label = "Default Folder"
 		defaultFolder.RescanIntervalS = 60
-		defaultFolder.MinDiskFreePct = 1
+		defaultFolder.MinDiskFree = config.Size{Value: 1, Unit: "%"}
 		defaultFolder.Devices = []config.FolderDeviceConfiguration{{DeviceID: myID}}
 		defaultFolder.AutoNormalize = true
 		defaultFolder.MaxConflicts = -1