New adaptation for LuCI openwrt-19.07 branch

Adapt to official  mainline  snapshot

test on LuCI Master git-20.033.77428-3d63732

v2.01

Fix display error, when not set password.
Notcie:
Only for openwrt 19.07 Stable and LuCI openwrt-19.07 branch (git-20.006.26738-35aa527).

v1.3: Adapt to official mainline snapshot

test on LuCI Master git-20.033.77428-3d63732

v1.2

Notice: please install luci-lib-fs_1.0-1_all.ipk first。
1.Minimal background file size.
2.Automatically detect the number of background images.
3.Delete unused image and file.

v1.1

Update README.md