Back macro won't interfere with holding backspace deleting multiple characters

[Ruzihm]

Closes #1583

Test case setup:

Edit TestGame's TestInterface.dmf and add a new macro to macro list:

• Action: Back
• Command: say hi

Test case:

1. Click on map then press backspace.
2. Click on text input then press backspace.
3. Click on text input, input asdf, then hold backspace for a few seconds.

Byond:

1. Command happens (xyz says: "hi" appears in output)
2. The command happens once.
3. Whole input gets emptied, then the command happens once.

OD Before PR:

1. Command happens
2. The command does not happen.
3. Only f gets removed. Command does not happen.

OD After PR:

1. Command Happens
2. The command does not happen
3. Whole input gets emptied. Command does not happen.

The remaining disparities here (in bold) basically require a couple RT changes. Making these changes to RT and rebuilding bring the test case to full parity.

1. A LineEdit that calls args.Handle() in KeyBindDown under different conditions - in this case, in backspace it would only be called when it actually removes character(s). Other controls would need similar treatment, for situations like writing on paper.
2. A change to UserInterfaceManager.OnUIKeyBindStateChanged where it can require args.Handled to also be true in order to return true.