ui/app: add a DataDir fallback to Go 1.12

o.UserConfigDir is introduced in Go 1.13, which is not released yet.
Add a Go 1.12 stopgab until then.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/datadir.go

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-// +build !android
+// +build !android,go1.13
 
 package app
 

ui/app/datadir_go112.go

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// +build !android,!go1.13
+
+package app
+
+import "os"
+
+func dataDir() (string, error) {
+	// Use a quick workaround until we can require go 1.13.
+	return os.UserHomeDir()
+}