Fix preview for files in projects

[BafDyce]

This patch aims to fix #13. When opening a project, each file-editor receives a pointer to the editor instance of the root file. After saving a non-root file of a project, Gummi will compile the pdf as if the root file was changed.

Note: This doesn't provide a live-preview for non-root files, but at least it shows the compiled pdf, without having to switch to the root-file editor.

I did test it a bit (on Linux only though), however since I didn't know the codebase at all before doing the change, I don't know whether I missed something.

[BafDyce]

Note: Today I discovered that this does not work yet when you just created the project. However, after restarting Gummi and opening the project via "Project -> Open Project" the preview functionality works as intended. As soon as I have time to investigate that further, I'll fix that.

[tchotchke87]

Would it be possible for you to produce a version of this patch that applies to the GTK3 port of Gummi (non-official)?

This is probably not where I should ask this, but I'm new to github. How do I apply this patch using Ubuntu to the version of gummi I can download from here?

[BafDyce]

I'm quite busy these days, however I'll try to find some time to look into this in the next few days. I don't know how much of the codebase changed, so it's either directly applicable or I need to rewrite some stuff.

I'll let you know once I know more.