This commit merges all ad-hoc file dialog code, such as the feature
where dialogs remember last location and format, and exposes it
through a common interface.
This commit also significantly improves Gtk dialog handling code.
This commit mostly just changes the settings code to be in line with
the rest of the platform abstractions, although it also fixes some
settings names to be consistent with others, and uses native bool
types where applicable.
This commit also makes settings-related operations much less
wasteful, not that it should matter.
This commit removes a large amount of redundant code that needed
to be kept in sync between platforms and also makes it much easier
to add new menu-related functionality since little to no platform
code needs to be altered anymore.
This commit also greatly improves code locality in context menu
handling by allowing context menu click handlers to be closures.
This commit temporarily introduces a SetMainMenu API, which is rather
hacky but only necessary until an abstraction for windows is added.
whitequark