Implement custom column manager for special locations

Custom locations like computer:// network:// and trash:// can have special properties that should be displayed on details view. Such examples include : "Storage size" and "Device type" for computer://, "date deleted" for trash://. Also, computer:// does not have properties like "date modified" and such. But current implemetation of thunar isn't that flexible with column model, so it does not have a notion of "custom column".

Possible issues

• ThunarWindow has to verify custom location to set up custom columns and mask invalid columns. But the actual logic to set up custom column settings should be somewhere else like thunar-column-model.c for code integrity.
• ThunarColumnModel does not know about custom location, so opening custom location could mess up previous column config data.
• ThunarColumnEditor does not know about custom location, so custom columns will appear in Configure Columns.... A user may be confused about this because these columns would not be enabled/disabled in normal locations. (This has already happened for Date Deleted)
• Do we need separate user column settings for custom locations? If then, how should it be handled?

Existing custom columns

• trash:// has its own custom column Date Deleted (thanks to Sergios - !70 (merged)).
• computer:// has custom column behavior for Name and Size (!92 (merged)).

Other related issues

• Columns for media tags (mp3 and flac) (#585)