Handle mouse buttons when the active form changes

[SuuperW]

Before 3eb9cb9 the state of mouse buttons was not checked by Input when the active form did not "want mouse" (meaning, clicking on that form should not result in controller inputs being pressed or hotkeys being triggered). After, they are checked and so no non-"ignored" mouse input event would be generated if you click on the main form when TAStudio was active.

This PR fixes that by having Input detect when the active form changes, and then clears its mouse state and generates new mouse events. This only happens when moving from a non-mouse-wanting form to a mouse-wanting form, which seems consistent with the behavior of 2.11 but might not be ideal. This is because clicking on TAStudio while the main form is active will result in a mouse input most of the time; Input sees the mouse button being pressed before Form.ActiveForm changes. If we clear mouse state and generate new events going the other way too then any controller inputs bound to the mouse button will be quickly released after ActiveForm changes, but the inputs will still happen. But even with this issue, this is an improvement over 2.11. In 2.11 if you click on TAStudio while the main form is active, any controller input bound to the mouse button will be pressed and will not be released when the mouse button is released. Now, it will be released when the mouse button is released.

I also had to change call sites of Input.ControlInputFocus, which previously was being called in forms' OnActivated and OnDeactivate methods. This doesn't make any sense to me, since the call to Input.ControlInputFocus in OnDeactivate is not required; mouse inputs won't happen when the form is active because Input checks which form is active. EDIT: It had to be changed because if the form does not remain in the wanting-mouse state, Input may see it does not want mouse (because ControlInputFocus had not yet been called in OnActivated) and thus not produce new mouse events when the active form switches. It's a race condition.

Please test, @RetroEdit.

Fixes #4717

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[RetroEdit]

Seems to have resolved the issue.