Add an input option to support sticky mouse buttons

[xuhdev]

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• Description:

X.org has an option called DragLockButtons. This enables simulating pressing down another mouse button.

For example, a user with many mouse buttons can specify one mouse key to serve as a sticky left-click key. The user pushes that button once and the left-click button remains pushed. Then the user can move mouse to select text. Finally, the user pushes the mouse button again to release the left-click button.

This is useful for many people with Repetitive Strain Injury (RSI), and trackball users who often can't select accurately when holding another button with the same hand.