File Manager fixes
Created by: xTibor
This pull request mainly contains rendering fixes for the File Manager.
- I introduced a Column struct to hold information about the column headers.
- The column widths are now initialized based on the width of the column header labels.
- The ".." entries are now accounted for the column width calculations.
- The ".." entries are now not shown at the scheme root folders.
- Added a 4px left margin to the icons.
- Changed the rendering of the up/down arrows. It's easier to render these glyphs using Unicode characters.