Expose tm_workspace_update() to plugins

As mentioned in the commit, calling tm_workspace_update() just once instead of performing parent (i.e. workspace) update in tm_source_file_update() every time source file is added makes a huge performance difference.

Before, it was impossible to generate tags for all files of the linux kernel size project (UI got completely frozen). Using the method above, it takes about 30s to generate all tags and about 3s to open/close file or find tag (which is a bit slow, but usable already).

I'll update my plugin (gproject) and also geanyprj, which does a similar thing, once this is merged.
nitpick: parent's -> the parent object's (ditto for the next edit below)