Provide a fast way to select resources from the project

[bjorn]

Often Tiled needs the user to pick a resource, like a tileset when adding one to the map or an image when setting up a tileset. Most of the time, those resources are expected to be located within one of the directories added to the project. There should be a quicker way to select those resources than going through the system file dialog.

The Defold editor provides a good example of how this might work, essentially re-using the "open file in project" popup with a file type filter added on top. Alternatively there could be a kind of search-enables drop-down.

I'm not sure if it makes sense to force people to set up a project at this point, but it probably should remain possible to select a resource with the system file dialog as well.