My players still use this extension some of the time to target, so I've updated it to fix the deprecated messages the players were getting in the chat window (and console).
The .ext in the 1st post has been updated to be v0.5 and is designed for FGU. The extension has been completely rewritten to bring more into line with the way the combat tracker is currently structured. It's built upon CoreRPG (I only test with 5E but it might ok for some other rule sets).
There are plenty of built-in ways for players to select or deselect targets
i) Ctrl-leftclick on an entry in the combat tracker (this is standard)
ii) Ctrl-leftclick on a token on the battlemap (this is standard). If you have several tokens to control leftclick the appropriate token 1st to select it (this is standard).
This extension adds some additional things (inside a DM-like targeting sub-section)
iii) a button on the player side to clear all targets
iv) a player way to target by dragging and dropping the targeting cross-hair onto an entry in the combat tracker.
v) a player way to target by dragging and dropping the targeting cross-hair onto a token on the battlemap.
vi) a player can deselect an individual target using a leftclick on the small icons in the combat tracker targeting subsection.