No progress bar for opening large files

[ahoneybun]

When opening large files in Gedit there is a progress bar for loading the file but COSMIC Edit simply does not start until it loads the file. From the outside this might look like the application is not working and could be confusing.

Cosmic-edit version:
0.1.0172623683224.04~9477a47

Steps to reproduce:

1. Open a large file in Gedit and COSMIC Edit
2. Notice the progress bar at the top of Gedit and the delay of COSMIC Edit

Expected behavior:

COSMIC Edit has a progress bar or give some information to the end user that it is loading the file.

[skygrango]

If the file is too large, it will take a long time to load and may appear to the user as stuck
I think the dynamic loading function needs to be implement in the future