Add Recent/Pinned category

[jeremypw]

Fixes #535

One of two new categories may be created, depending on the build options

If the Zeitgeist option is compiled in:

• Recently used apps. Shows apps used in the last [4] weeks in descending order of usage as determined by Zeitgeist. This uses the pre-existing (but hitherto unused) code related to app popularity but simplifies the calculation of the popularity measure.

If there is no Zeitgeist:

• Pinned apps - this is still useful for those of us who dislike using the dock or have a window fullscreened/maximised so the dock does not appear on screen.

It would be possible to have a further build or system option to control showing of the Pinned category but that is left for now.

Unless another category has been previously selected, the Recent/Pinned category is selected by default.

TODO

• [x] Make sure app popularities have finished initialising when first shown after restart.
• [x] Update the Recently used category after an app has been used. At the moment the indicator has to be restarted.

[danirabbit]

I don't think this addresses #535. That report is about frequently used apps, not pinned apps

[jeremypw]

@danirabbit Well, there is overlap between the two sets (pinned apps are likely include some of those recently used). The PR is not yet finished.