Addon Manager: Begin UI redesign

Migrate to a UI that looks more like other software package managers, giving
more space to each addon by hiding the list when an addon is selected, and
providing a "back" button to get back.

Implements a two-style view option for the list of addons: expanded and
condensed, via a delegate that provides the drawing function for each row in
the table based on two different widget designs.

src/Mod/AddonManager/addonmanager_metadata.py

@@ -52,7 +52,7 @@ class MetadataDownloadWorker(QObject):
         super().__init__(parent)
         self.repo = repo
         self.index = index
-        self.store = os.path.join(FreeCAD.getUserAppDataDir(), "AddonManager", "PackageMetadata")
+        self.store = os.path.join(FreeCAD.getUserCachePath(), "AddonManager", "PackageMetadata")
         self.last_sha1 = ""
         self.url = self.repo.metadata_url