There's no point in having a try block when the condition is false, so
invert the if & try to avoid handling exceptions for nothing

Instead, fetch the row fields, and update the mrl only. Otherwise, we
can't edit the Task schema as we'd fail to load any task using the
updated task class with an outdated database schema

This is the only class doing it, it clutters the code for not much
gain, and would enforce unique constraint we don't need for the backup
table

fsDir can't be null, as createDirectory always returns an IDirectory
instance

Fix #213

They were missing an 'S'

codec is already assigned below, in a correct way, while this is
assigning an integer to an std::string

To avoid passing duplicated IDs to the applications
Fix #180

Instead of reimplementing all callbacks

To ensure we don't read out of bounds.
This isn't fixing any actual (known) defect, but it makes more sense
from a logical point of view, and fixes a warning with cppcheck

In this scope, oldThumbnail is always nullptr

As is the regular std::mutex implementation

size() might take linear time to count all the items, while we're only
interested in the empty state.