Add a find in files panel on the GUI

[narnaud]

This is the next step after #21.

The idea is to have a special panel for find in files, with the list of results displayed. Clicking on a result will open the document and select the word.

Note that results won't be updated if a document is changed after the find (for now).

Should be something like we have in VSCode... if we use rg for finding in files, we could display a bit of context.