Feature: Typed Source

[LuanHimmlisch]

This PR is meant to refactor the code relating to the state of the Front actions taken by the user, also known as the source type. Previously, this source was managed as a simple string that prevents Intellisense from working, and allows human error.

This PR does the following to fix the mentioned errors:

• Adds a WeblaborMx\Front\Source class, that is meant to be used as an Enum compatible with PHP 7.1.
• Adds constants to enumerate the different cases.
• Adds various methods to check whether a source is certain case, with the added utilities of isForm(), isServerSide() and isClientSide().
• Adds validations to ensure the values of the source are valid according to the constants.
• Decouples the session() out of the Sourceable trait, in order to reuse it inside the Input class.
• Makes the Source class Stringable so it can be cast to string and used as it was.
• Marks the use of the $source variable as @deprecated to prevent the usage.

With this PR, the value of the source is always known, through enumeration, validation and IDE autocompletion.

[LuanHimmlisch]

Notable code:

No magic strings, everything is enumerated.

Helper functions such as isForm.

Removed usage of $source directly. Using class methods instead.

This allows for different ways to structure the resources, instead of hiding the inputs. Specially useful for larger resources.

[skalero01]

Please update the readme with the changes