It's impossible to add external engines on Linux if their filename doesn't end with .exe

[marmistrz]

The file picker only allows .exe files, however, the standard convention on Unixes is that executables don't have any extension.

I tried to contribute a fix, but I was stopped by language barriers, see https://github.com/lukasmonk/lucaschess/issues/131

[lukasmonk]

PantallaMotores.py, line 435

-     exeMotor = QTUtil2.leeFichero(wowner, folderEngines if folderEngines else ".", "%s EXE (*.exe)" % _("File"), _("Engine"))

+    exeMotor = QTUtil2.leeFichero(wowner, folderEngines if folderEngines else ".", "%s (*.*)" % _("File"), _("Engine"))