textscreen: Activate window for file select.

This pops up the file select dialog immediately rather than waiting for
the user to activate the application. But really the current file select
code for OS X based on AppleScript is garbage and should just be
replaced.

textscreen/txt_fileselect.c

@@ -329,6 +329,7 @@ char *TXT_SelectFile(char *window_title, char **extensions)
 
 #define APPLESCRIPT_WRAPPER \
     "tell application (path to frontmost application as text)\n" \
+    "    activate\n" \
     "    set theFile to (%s)\n" \
     "    copy POSIX path of theFile to stdout\n" \
     "end tell\n"