Editor send repeat http requests when first open "Templates" tab

[qichunren]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
OS: Ubuntu 16.04
Gotdot version: aa6772d

Issue description:

After click "Tempaltes" tab, I found it emit 2 same http request "https://godotengine.org/asset-library/api" at first time.
Expect: It should send one http request.

I don't know why NOTIFICATION_VISIBILITY_CHANGED signal emit twice.

Steps to reproduce:

Start ./bin/godot.x11.tools.64 to run godot, then click "Templates" tab.

Console Log:

:~/code/godot$ ./bin/godot.x11.tools.64
No touch devices found
OpenGL ES 3.0 Renderer: Gallium 0.4 on llvmpipe (LLVM 4.0, 256 bits)
GLES3: max ubo light: 409
GLES3: max ubo reflections: 455, ubo size: 144
ARVR: Registered interface: Native mobile
..https://godotengine.org/asset-library/api
..https://godotengine.org/asset-library/api
CERT STR: /CN=godotengine.org
VALID: 1
CONNECTION RESULT: 1
cert_ok: 1
response: 0 code: 200
{
"categories": [ {
"id": "8",
"name": "Templates",
"type": "1"
}, {
"id": "9",
"name": "Projects",
"type": "1"
}, {
"id": "10",
"name": "Demos",
"type": "1"
} ]
}
CERT STR: /CN=godotengine.org
VALID: 1
CONNECTION RESULT: 1
cert_ok: 1
response: 0 code: 200
{
"page": 0.0,
"page_length": 10.0,
"pages": 1.0,
"result": [ {
"asset_id": "89",
"author": "Punpun",
"author_id": "224",
"category": "Projects",
"category_id": "9",
"cost": "MIT",
"godot_version": "3.0",
"icon_url": "https://raw.githubusercontent.com/Lohan120/GodotIsoCustomPathfinder/Godot-3.0/icon.png",
"modify_date": "2017-12-12 09:29:09",
"rating": "0",
"support_level": "community",
"title": "TileBased Custom Pathfinder",
"version": "2",
"version_string": "1.1"
}, {
"asset_id": "99",
"author": "DmitriySalnikov",
"author_id": "251",
"category": "Templates",
"category_id": "8",
"cost": "MIT",
"godot_version": "3.0",
"icon_url": "https://github.com/DmitriySalnikov/godot-console/blob/master/icon256.png?raw=true",
"modify_date": "2017-12-12 09:34:18",
"rating": "0",
"support_level": "community",
"title": "In-game Debug Console",
"version": "1",
"version_string": "1.1.1"
} ],
"total_items": 2.0
}
REQUEST ICON FOR: 41355526
REQUEST ICON FOR: 41355526
CERT STR: /C=US/ST=California/L=San Francisco/O=GitHub, Inc./CN=github.com
VALID: 1
CONNECTION RESULT: 1
cert_ok: 1
CERT STR: /businessCategory=Private Organization/jurisdictionC=US/jurisdictionST=Delaware/serialNumber=5157550/street=88 Colin P Kelly, Jr Street/postalCode=94107/C=US/ST=California/L=San Francisco/O=GitHub, Inc./CN=github.com
VALID: 1
CONNECTION RESULT: 1
cert_ok: 1
GOT IMAGE YAY!
CERT STR: /businessCategory=Private Organization/jurisdictionC=US/jurisdictionST=Delaware/serialNumber=5157550/street=88 Colin P Kelly, Jr Street/postalCode=94107/C=US/ST=California/L=San Francisco/O=GitHub, Inc./CN=github.com
VALID: 1
CONNECTION RESULT: 1
cert_ok: 1
CERT STR: /C=US/ST=California/L=San Francisco/O=GitHub, Inc./CN=github.com
VALID: 1
CONNECTION RESULT: 1
cert_ok: 1
GOT IMAGE YAY!
^C

Link to minimal example project:

[qichunren]

This issue should be closed as 0afdc5c commit.