Add L3 and R3 buttons to full OSC layout

[lyndsay9968]

Is your feature request related to a problem? Please describe.
Currently, the L3 and R3 keys are unavailable, making some games unplayable.

Describe the solution you'd like
Full OSC layout should have L3 and R3 buttons.

[TrueZhuangJia]

double click on-screen sticks quickly to trigger L3 & R3

[lyndsay9968]

OK, but the ability to add such buttons would be more convenient in many cases. They do not have to be enabled by default. The ability to add them by key code is enough, no need to add a menu item. #32

[tignioj]

快速双击摇杆不太灵敏，我在玩大表哥的时候，有个操作需要同时按下L3和R3，就不太好触发