Allow PIN lock for apps/app drawer

[neckcen]

See discussion in issue #24 for reasoning, might be combined with #10 (I believe that's how smartlauncher does it).

[Lonami]

How should this work? Some applications would be locked. Does it ask for the pin when opening? Or you enter the pin via the settings fragment and then it temporary shows the locked ones?

[neckcen]

I don't think pin protecting apps in the launcher is really useful as there are many ways users could still access them (share, last used, ...).

Therefore I would personally go with the second solution: some apps can be hidden, showing hidden apps can be enabled in the settings and settings can be locked with a pin.