MythThemedMenu: Add dependsexec option to the main menu

This option allows for menu entries that will only be visible if a particular
executable is found. For example the menu entry below will add an XBMC menu
entry if the XBMC executable is found at /usr/bin/xbmc.

<button>
  <type>TV</type>
  <text>XBMC</text>
  <description>Launch XBMC</description>
  <action>EXEC xbmc</action>
  <dependsexec>/usr/bin/xbmc</dependsexec>
</button>

Fixes #12021.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>