[5.x] Add increment/decrement methods to ContainsData

[duncanmcclean]

This pull request adds increment and decrement methods to the ContainsData trait, which is used pretty much everywhere.

So, instead of needing to do this:

$entry->set('inventory', $entry->get('inventory') + 1);

You can now do this:

$entry->increment('inventory');

Not related to an existing feature request, but something I thought might be useful last night while working on a personal project.