Display total number of packages in all three main views

**Is your feature request related to a problem? Please describe.**

There is a very basic piece of information missing in main tabs/views (_Explore_, _Installed_ and _Updates_) - the total count of packages in the current view. For example there is no way to see the number of installed apps.

**Describe the solution you'd like**

For all three main views to provide the total package count. In the _Explore_ tab meaning the number of packages given the current search filters.