Open up some RepositoryManager sections

Since RepositoryManager is not an interface, it needs to be extended by subclassing, so its main components need to be protected, rather than private.
Also remove final modifier for the internal map
Allow setting, and overriding of setInitializedRepositories

This allows users to fully override the current behaviour in subclasses
SES-2092 Removed default PNG images, and replaced with nearly identical CSS specifications.
SES-2092 Tweaked the gradient to be closer to PNG gradient.
SES-2092 These changes rely on IE 10+, so keeping stylesheets supporting IE 5.5-7 is silly.
SES-2092 Added pre-IE10 filter property to get something close to the desired gradient on those IE versions.
SES-2092 Restored hr_menu PNG, since it actually contributed to the look.
SES-1652 trivial null check
Merged in dvisser/sesame/SES-2092-replace-png-with-css (pull request #304)

SES-2092 Replaced background PNG files with CSS color and gradient properties
SES-1652 fixed problem with calling commit/rollback when no transaction
is active.
Merge branch '2.7.x' into 2.8.x
[maven-release-plugin] prepare release sesame-2.8.0-beta2
[maven-release-plugin] prepare for next development iteration