Download custom appliance symbols from GitHub

Fix symbol cache issue. Ref https://github.com/GNS3/gns3-gui/issues/2671
Fix temporary directory for symbols was not deleted
Fix temporary appliance file was not deleted

tests/controller/test_node.py

@@ -259,16 +259,15 @@ def test_symbol(node, symbols_dir, controller):
     Change symbol should change the node size
     """
 
-    controller.symbols.theme = "Classic"
-    node.symbol = ":/symbols/dslam.svg"
-    assert node.symbol == ":/symbols/dslam.svg"
+    node.symbol = ":/symbols/classic/dslam.svg"
+    assert node.symbol == ":/symbols/classic/dslam.svg"
     assert node.width == 50
     assert node.height == 53
     assert node.label["x"] is None
     assert node.label["y"] == -40
 
-    node.symbol = ":/symbols/cloud.svg"
-    assert node.symbol == ":/symbols/cloud.svg"
+    node.symbol = ":/symbols/classic/cloud.svg"
+    assert node.symbol == ":/symbols/classic/cloud.svg"
     assert node.width == 159
     assert node.height == 71
     assert node.label["x"] is None

tests/controller/test_symbols.py

@@ -31,13 +31,15 @@ def test_list(symbols_dir):
     symbols = Symbols()
     assert {
         'symbol_id': ':/symbols/classic/firewall.svg',
-        'filename': 'classic/firewall.svg',
-        'builtin': True
+        'filename': 'firewall.svg',
+        'builtin': True,
+        'theme': 'Classic'
     } in symbols.list()
     assert {
         'symbol_id': 'linux.svg',
         'filename': 'linux.svg',
-        'builtin': False
+        'builtin': False,
+        'theme': 'Custom symbols'
     } in symbols.list()