LaunchServer: Make all file handlers configurable including directories

This commit gets rid of hard coded file handlers in Launcher.cpp in
favor of using values in the LaunchServer.ini config file.

The previous commit adds checks for the existence of handler programs
while registering handlers. This commit takes advantage of that and
ensures that LaunchServer will not attempt to open a file with a
nonexistent program and can properly report failure before spawning a
new child process.

Resolves #8120

Base/home/anon/.config/LaunchServer.ini

@@ -17,6 +17,7 @@ font=/bin/FontEditor
 sheets=/bin/Spreadsheet
 gml=/bin/Playground
 pdf=/bin/PDFViewer
+directory=/bin/FileManager
 *=/bin/TextEditor
 
 [Protocol]