Feature: Add Vunit menu to Modelsim GUI

[krishnan-gopal]

In the default gui.do file, it would be good to have a VUNIT menu to select the commonly used VUnit TCL commands like compile, restart and so on. This makes things a lot easier while using the GUI
This TCL code implements a VUNIT menu along with menu items for the Vunit tasks, when loaded in vsim GUI

add_menu "" VUNIT
add_menuitem "" VUNIT "vunit user init" vunit_user_init
add_menuitem "" VUNIT "vunit compile" vunit_compile
add_menuitem "" VUNIT "vunit run" vunit_run
add_menuitem "" VUNIT "vunit restart" vunit_restart
add_menuitem "" VUNIT "vunit help" vunit_help