trackmouse: Default to combination of 3 modifier keys (Meta, Ctrl, Alt)

Avoid accidental activation of the "Track Mouse" animation while using some standard shortcuts.

Examples:

1. Meta+Ctrl+Arrowkeys is currently used to switch desktops (since commit #e3982892)
2. Meta+Ctrl+MouseWheel is currently used for "Zoom In" and "Zoom Out" actions (since commit #5b3529ac)
3. Meta+Ctrl+T is used for the "Thumbnails aside" effect
4. Meta+Ctrl+A is used for the "Activate Window Demanding Attention" action

In all of these cases, the "Track Mouse" animation is unexpectedly triggered due to the usage of "Meta+Ctrl".

We can avoid such misbehavior by adding one more modifier ("Alt") by default.