Consolidate handling of TGFileBrowser* TEveBrowser::fFileBrowser member, see
ROOT-5294.

Add 'SetFileBrowser(TGFileBrowser*)' method.

Add bool flag to 'TGFileBrowser* MakeFileBrowser(Bool_t make_default=kFALSE)'.

(cherry picked from v5-34-00-patches, commit e58c8560732f44d8d930ec861fda135e092de428)