Use container margins in content container for topbar.

Wrappers are added to layout areas to keep content on top of styled
generated content (for adding backgrounds).

The props for styling areas of layouts have been added, with sensible
low-specificity default styles added.

Examples are for ease of use of the library.

The existence of the TopBar widget depends on some props related to it.

The CSS variables have been documented to help with guiding the user
for configuration.

The components have been adjusted to define variables internally. They
will use internal defaults but will inherit their ascendant's CSS
variables.

Add files from TSDX.