Allow locking launchers

[leolost2605]

Fixes #334

Has to be exposed in the settings still.

[teamcons]

Some nitpick - My apologies. I am looking at your PR positively
The formulation is slightly misleading, the items are not really locked - people can still add or remove them. It is more "Disable removal of pinned apps with a drag and drop"

In the future, with a dock visible in multitasking, a case could be made that people would want as feature to drag and drop an icon on a desktop to open the app there.
So i would be partial to instead have an enum allowing either to remove (now), do nothing (the locked behaviour), or open on said desktop

[leolost2605]

@teamcons that's a good idea I updated it with an enum :)

[zeebok]

Conflict to resolve!