[!!!][TASK] Use request type constants everywhere

Each entry point (application) now defines the actual
request type at a very early point of the request, so
each call within TYPO3 can use the predefined constants
at all times.

Previously this was done within the Bootstrap after loading
LocalConfiguration.php etc, and by evaluating other
parameters to detect which RequestType should be used.

By directly setting the option within each application, the
constants "TYPO3_enterInstallScript" and "TYPO3_cliMode"
can be removed, as well as $GLOBALS['TYPO3_AJAX'].

Resolves: #72368
Releases: master
Change-Id: I5080e425d70cb6d4c9a9573dbc20216c93cd3332
Reviewed-on: https://review.typo3.org/45379


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>