Implements custom, VFS-based map load/save dialogs for Atlas (replaces
broken native file dialogs), fixes#631, #889.
Fixes map loading/saving to handle arbitrary subdirectories for better
organization.
Adds default settings to Atlas player panel, fixes#1872. Each setting
now has a checkbox to choose whether it should be saved with the map
(avoids writing lots of useless default data for each map).
Adds map preview setting to Atlas, refs #1745.
Cleans up and simplifies some duplicate code.
Fixes optional serialization performance test.
This was SVN commit r13938.