[CLEANUP] Code Cleanup of AbstractPlugin

AbstractPlugin serves as a base for non-extbase plugins.
In order to allow for a notice free environment and no code warnings
within TYPO3 CMS AbstractPlugin should:
* provide a central property for Database access (DatabaseConnection)
* provide a central property to access GLOBALS['TSFE']
These properties will then aid both us in the CMS core development as
well as extension authors by supplying code completion in IDEs like
PHPStorm or alike.

Resolves: #62662
Releases: master
Change-Id: I6a698b2481f33643ab800c7152f710f4ed6706a5
Reviewed-on: http://review.typo3.org/33762
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>