[Dependency Injection] Detect proper web root path based on Symfony kernel version

[robfrawley]

Q	A
Branch?	2.0
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#1037, #1041, #1042, #1039
License	MIT
Doc PR	Included

Enable dynamic default configuration values for the following configuration entries:

• liip_imagine.loaders.<name>.filesystem.data_root
• liip_imagine.resolvers.<name>.web_path.web_root

The folder layout for the web root directory changed between Symfony 3.x (%kernel.project_dir%/web) and 4.x (%kernel.project_dir%/public). This PR implements an alternate to #1039 as discussed there, where the DI factories dynamically assign the default values based on the Symfony kernel VERSION_ID. All documentation and tests were also amended to accommodate the aforementioned changed.

Additional changes, outside the scope of this titled PR, are:

• Simplification of the SymfonyFramework::kernelVersionCompare() method implementation.
• Adding proper type hints to SymfonyFramework.

[lsmith77]

cool .. yeah I guess we will just keep adding type hints as we touch code for now .. and maybe at some point do a concerted effort. there might be some tools out there that can convert phpdoc to typehints.