Scrub newlines from file names when displaying them

The POSIX specification for file names allows the use of the newline
character for file names, but Font::write treats it as a control
character. It was possible for a malicious user to place a file whose
name contained a newline character to trick the user into selecting a
file that he or she did not intend to select, or to fill the screen
with garbage text due to overlapping.
Display the current path in file selectors

Whenever it makes sense, the current path is shown in the place of the
Dialog's subtitle.
* In FileDialog, used by the Explorer link to find executables, the
  path is shown under "File Dialog". Previously, this was the subtitle,
  "Select an application".
* In ImageDialog, providing the icon selection for links, the path is
  shown under "Image Dialog". Previously, this was the subtitle,
  "Select an icon for this link".
* In DirDialog, providing the initial directory in a link's file
  selector, the path is shown under "Directory Browser". Previously,
  this was the subtitle, "Directory to scan for the selector".
* In WallpaperDialog, used by the Wallpaper link, no path is shown.
  The subtitle, "Select a wallpaper from the list", is still displayed.
Make the titles more descriptive in file selectors

* In FileDialog, used by the Explorer link to find executables, the
  title is now "Select an application" instead of "File Dialog".
* In ImageDialog, providing the icon selection for links, the title is
  now "Select an icon for this link" instead of "Image Dialog".
* In DirDialog, providing the initial directory in a link's file
  selector, the title is now "Directory to scan for the selector"
  instead of "Directory Browser".
* In WallpaperDialog, used by the Wallpaper link, the title is now
  "Select a wallpaper from the list" and there is no subtitle. This
  allows a bit more of the wallpaper to show through.
Fix integer underflow in BrowseDialog::adjustSelection

Moving the selection cursor into the bottommost quarter of the screen
caused an integer underflow if the entire list fits in one page.

This commit forces firstElement to be 0 in that case.