Startup Applications - Some adjustments

On Startup Applications Panel:

• Fixed "+" button to be always at the end
  When adding a new item, the "+" button was staying between the existing items and the newly added

• Fixed item list to always respect alphabetic order
  As the new item was being added to the end, after reopen Tweaks, the order of initialized items was changing
  I don't know exactly why, but the list always show the reversed order, so I reversed the order when adding the programs

• Changed the application selection window to have the default 600px height instead of 300px
  I don't know if there was a reason for the small size, but for me that seems very annoying.

• Removed duplicated code

This is my first PR on a GNOME project, if there is something wrong please let me know