screenedge: reduce default activation delay

Currently there is a 150ms delay before screen edge actions and
hotcorners are triggered; the cursor has to spend that much time there
before they'll activate. This is good for preventing accidental
activations, but also prevents and delays deliberate activations, which
are likely to be more annoying for the user.

To alleviate this, the delay is reduced to 75ms, which still prevents
most of the accidental activations in my testing, while making
deliberate activations faster and easier.


(cherry picked from commit e963ae09)