Support selecting storage location in editor file browser

Initially the file browser (maps, images, sound) shows the files from all storage locations combined like before when opening a file for reading. When saving a map, only the save storage location is shown and used like before.

The folder ".." is now also shown in the root folder, to navigate up to the storage location selection, if more than one storage location is present where a "maps"/"mapres" folder exists (whichever the dialog is currently showing). Only the locations where one of those folders exists are shown in the storage location selection. Additionally "All combined" can be selected to go back to the combined view.

The "Show directory" button behavior is adjusted so that the folder that contains the currently selected item is opened. When the storage location selection is shown, the button opens the selected storage location instead.

When navigating to the parent folder, the previous folder is now initially selected instead of resetting the selection.

Add a link to the "themes" folder same as the link to the "downloadedmaps" folder to allow easier access to the themes with the map editor.

The "folder tree" icon which is used for the ".." folder is now also used for the folder links in the storage location selection and for the links to the "downloadedmaps" and "themes" folders.

Always sort links above normal folders but below the ".." folder.

Fix alignment of font icons by using the correct flags.