ESP-IDF Release v4.0 Support & Open PR Consolidation

[jeremyjh]

I've merged all of the open PRs in my branch, but also contributed some changes to make this library compatible with the latest changes in ESP-IDF and fully configurable through idf.py menuconfig.

I'm hopeful this could be merged into the upstream repository but I'll also continue to take PRs at https://github.com/jeremyjh/ESP32_TFT_library .

Other than required updates for 4.0, the motivation for these changes is to improve the usability of this repo as an ESP library component rather than the basis of a new project.

This now builds its subcomponents with idf_register_component. Instructions for including as a library to an existing project have been added, along with updates for building and running the examples with idf.py.

All the configuration that is relevant to using this project as a library (e.g. SPIFFs, predefined screen configs) have been moved into the component folders, rather than being configured in the Demo project.

It is now possible to configure all the pins individually through menuconfig, and instructions about modifying the headers directly have been removed.

I also prefixed all global variables with tft_ that were not already so prefixed. This is perhaps the biggest breaking change for existing users, but I feel it is for the best as it makes usage of these variables in application code much more obvious, and reduces the possibility of name collision with other libraries.

The other breaking change is the renaming of the Example configs. These were already used in the library proper and referred to as predefines so I updated the name itself to reflect that along with moving them into the library's Kconfig. To update, users just need to run idf.py menuconfig.