Deprecate ExoJob

ExoJob is a wrapper class for asynchronous jobs and Thunar depends heavily on it. But it has critical flaws that hinders Thunar from achieving true asynchronous operation.

1. ExoJob depends on GIOScheduler, which is deprecated as of year 2013

GIOScheduler is deprecated as of GLib 2.36. GTask replaces this one and provides an easy way to build an asynchronous operation and this also can be used to create synchronous version of an asynchronous operation. Since ExoJob depends on GIOScheduler, it does not provide a wrapper for methods of GTask.

2. Task priority of an ExoJob is always G_PRIORITY_HIGH

Which means, every ExoJobs and its derivatives are always top priority. Its priority is too high that it supercedes everything including timeouts, signal calls, and lo and behold, user interactions. Simply starting few dozens of them would make applications stutter. Setting this priority to somewhat lower would help this problem, but changing priority may cause heisenbugs. It should provide a way to set priority anyways.

3. Identity as "the class that handles asynchronous operations" and "an asynchronous operation itself" are mixed up

ExoJob by itself represents an asynchronous operation, but its subclasses in Thunar are enormous classes that manage asynchronous operations. This does not get along with the Glib/GTK way, which is chaining asynchronous operations to build new one. So it should be divided to two parts. The former could be replaced with GTask or its sub-class (let me call it ExoTask) and the latter could use ThunarJob for its pointer.

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To summarize: ExoJob might have been a good wrapper for GIOScheduler, but it is quite outdated for its job and could be replaced with GTask.