[NO SQUASH] IrrlichtMt: CGUIComboBox improvements

[SmallJoker]

Does not fix any reported issue.

Commit 1: Ensures that getText() always returns a valid pointer
Commit 2: Removal of superfluous/unused input handling code + click->drag->release selection (I can split this into a separate commit if desired)

To do

This PR is Ready for Review.

How to test

1. Open a formspec with a dropdown
2. Use the dropdown in whatever ways possible (scroll, click, drag&click)
3. With the test code below:
   1. Have an ItemStack in the first few slots
   2. Open the dropdown
   3. Right-click on the list options
   4. The ItemStack must not be picked up (event captured)

Some devtest test code:

/lua core.show_formspec("singleplayer", "test", "size[8,5]list[current_player;main;0,1;8,4;0]dropdown[0.25,0.2;2,0.8;test;Item 1,Item 2;0;]")