This Progressive widget "renders" tasks progressively, i.e. it returns   control to the browser periodically possibly allowing user interaction   prior to completion of the full task..     The concept "render" is used quite loosely, in that it could be building   part of a GUI or it could be loading the next successive portion of a table,   or it could be doing any partial task which requires periodically returning   control to the browser to allow user interaction.     This is a general purpose widget that happens to come packaged with some   useful renderers:       - A progressive loader, that periodically shows the GUI state as more of       the gui is being built;       - A table renderer that produces a table very similar in appearance to       qx.ui.table.Table but with a different set of features.  In particular,       this table allows variable row height.