Extra custom items for FmFolderModel.

The commit adds APIs fm_folder_model_extra_file_add() and
fm_folder_model_extra_file_remove() which add and remove some custom
FmFileInfo item into FmFolderModel. Those items aren't counted as
items from FmFolder associated with the model and will be not removed
if FmFolder is replaced. New items can be inserted into the model by
different ways:
- before all items of the FmFolder;
- mixed into items of the FmFolder by sorting rules;
- after all items of the FmFolder.
Save option 'drop_default_action' in more portable way.

If we save the option not as enum number but as text then it will be
well portable and safe if the enum will be changed sometime.
Document versions of appearance of new members of FmConfig structure.
New module: gtk_file_prop:inode/x-shortcut, it supports Type=Link desktop files.

The icon, name, and hidden attributes are currently supported to edit.
Item LinkTo belongs to PCManFM and SendTo is rather a category.

The Link item is similar to CopyTo or MoveTo and therefore better to be in PCManFM.
The "Send to" is a category which is already present in meny as AddBookmark or
Compress. It is better to just allow extensions to add items there instead.
Since both those items were never implemented, this commit just removes unused text.
Add MoveCategory and SendToCategory placeholders info FmFileMenu.

The AddBookmark and Compress moved into SendToCategory where they belongs.