Provide a fast way to select resources from the project

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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.

The issue names no files, tests, or entry points. Start by tracing the current system file-dialog flow for selecting tilesets and images, then compare it with the described project-file popup or searchable dropdown. Done means project resources can be selected faster while the system file dialog remains available.

Written by the indexing model from the issue text.