Debugger: fix UI text capitalization and punctuation

Added full stops to ends of messages.
Used book-style capitalization for message box titles.
Wrote "operating system" in lower case.

Change-Id: I8cee1a1c7f4ae55952daa507395845feae203ab9
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/debugger/terminal.cpp

@@ -90,7 +90,7 @@ void Terminal::setup()
         return;
     }
     if (!S_ISCHR(s.st_mode)) {
-        error(tr("Terminal: Slave is no character device"));
+        error(tr("Terminal: Slave is no character device."));
         return;
     }