Addon Manager: Create NetworkManager class

To enable single-login authenticated proxy use, and simplified multi-threaded
network accesses, this commit adds a new wrapper around a QNetworkAccessManager
and includes a global instantiation of the class intended to exist for the
lifetime of the program. This instance can be used to enqueue any number of
network requests, which the manager will send out to the networking subsystem
in an appropriate manner.

src/Mod/AddonManager/CMakeLists.txt

@@ -8,7 +8,6 @@ SET(AddonManager_SRCS
     AddonManager.py
     AddonManagerRepo.py
     addonmanager_macro.py
-    addonmanager_metadata.py
     addonmanager_utilities.py
     addonmanager_workers.py
     AddonManager.ui
@@ -18,6 +17,7 @@ SET(AddonManager_SRCS
     compact_view.py
     dependency_resolution_dialog.ui
     expanded_view.py
+    NetworkManager.py
     package_list.py
     package_details.py
 )