[LYN-2522] Filtering for gem catalog (#867)

* Added sort filter proxy model for gem model that can filter based on name, gem origin, supported platform, features and/or types.
* Added new filter pane on the left with several filter categories for gem origin, type, platform and feature.
* Added filter category widget which is a collapsable generalized checkbox group that can interact with the proxy model and thus control filtering.
* Removed fixed size of the project manager. The application should always be resizable.

Code/Tools/ProjectManager/Source/ProjectManagerWindow.ui

@@ -11,7 +11,7 @@
    </rect>
   </property>
   <property name="sizePolicy">
-   <sizepolicy hsizetype="Fixed" vsizetype="Fixed">
+   <sizepolicy hsizetype="Preferred" vsizetype="Preferred">
     <horstretch>0</horstretch>
     <verstretch>0</verstretch>
    </sizepolicy>