Introduced custom application launch text

This feature is meant for applications that require displaying different text than "Launching [app name]" when executed from gmenu2x, for example the "Poweroff" or "Reboot" scripts.

The custom launch text string is taken from:
"X-OD-LaunchMsg=" for OPK entries
"launchmsg=" for non-OPK entries

Language specific strings are also supported:
"X-OD-LaunchMsg[lng]="
"launchmsg[lng]="

Note: Due to the necessity to build a custom rootfs image in order to test the non-OPK menu entries, only OPK method was tested.