Add new API functions to get and set the visibility of table headers.

[szanni]

Proposal for two new API additions to un/hide table headers:

int uiTableHeaderVisible(uiTable *t);
void uiTableHeaderSetVisible(uiTable *t, int visible);

@nowind is this similar to what you had in mind with your comment in #310?

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Added API functions: uiTableHeaderVisible() to determine whether the table header is visible. uiTableHeaderSetVisible() to set the visibility of the table header.

Implementation provided for unix, darwin, and windows.

Notes: as darwin does not provide an API for hiding or recreating the table header I opted for saving a reference and restoring that when the visibility is set back to true. Setting the header to nil to hide it is the suggested method for hiding the header according to the docs.