Improvements to the File switcher

See original GitHub issue

I’ve been using VSCode a lot these days and I find its file switcher very neat and well designed:

seleccion_001

in comparison to our own:

seleccion_004

So I’d like to improve it for Spyder 4 with @juanis2112’s help. These are my suggestions:

Use a single row to show a file name and its path (instead of two, which is what we have right now).
Use the same font size for file names as the one used in the symbol switcher (@juanis2112, you can get that one by pressing Cmd+Alt+P). Also, use a font size one point smaller than that one for paths.
Use a grey color for paths instead of italics.
Make the switcher taller (look how many files are shown in VSCode in comparison to our own switcher). We should see at most 15 files on it.
Use the newly introduced file type icons to show them to the left of each file name (instead of the generic text file icon).
Remove the section header rows we have right now (Editor/Notebook). Instead, show those headers in blue to the right of the first file in the corresponding section. Also show an horizontal line when the section finishes.
Add a new section called Recent files to the switcher that shows the last 5 or 10 opened files that are not present in the Editor.
Don’t show a file in the Editor immediately after it’s selected in the switcher. Instead, users will have to press Enter to show a file from now on. However, the symbol switcher should keep working as it is doing it right now.
Show a place holder text in the switcher text field to explain users how to use it (like the one in VSCode: Escriba '?' para obtener ayuda ...). In our case, it could be something like Write some text to select among the currently opened files or consoles.
Improve paths shortening taking into account the dynamic size of the font