Show app download queue in main app list

As per the mockups in #592 (closed), F-Droid should be able to show the queue of apps awaiting download in the main list of apps. Thsi should be either in the Installed or the Updates tab. Some comments in #592 (closed) suggested Updates tab, others Installed. I'm leaning towards Installed, because we can remove the number from after the word "Installed" and instead only show that number if there is a download queue. Then the number in both tabs indicate that there is a number of apks for which a user action should be performed:

• The number on the Updates tab indicates there is apps worth looking at in order to update.
• The number on the Installed tab indicates there is apps that are downloaded and ready to install, but require the user to press a button to do so.

This will be done in multiple iterations.

The first iteration will just show the apps in the queue at the top of the "Installed" app list, and then specify whether it is "Downloading" or "Waiting for Download..." instead of the app summary.

The second iteration will include progress bars, so that the pending apps can have an indeterminate progress bar shown, and the downloading app can show legitimate progress.

A third iteration would show an "Install" button inline in the app list for apps which have been downloaded and are ready to be installed.