Add a toggle breakpoint button into the control buttons widget

[xusheng6]

There are several ways to set a breakpoint:

1. Right click, "Plugins" -> "Debugger" -> "Toggle Breakpoint"
2. Press F2
3. In the "Debugger" main window menu, click "Toggle Breakpoint"
4. In the breakpoint tab of the debugger sidebar widget, right click, "Toggle Breakpoint", and then type in the address
5. Use the API

Although the best way to do it is 2), lot of people only know about 1) and complain it is inconvenient to set a breakpoint. Which is reasonable if they have to go down three levels of menu to reach it. There are some other ways to make toggle breakpoint more discoverable:

1. #133
2. Put a breakpoint icon in the controls button widget

[xusheng6]

Besides, I can put the keybinding F2 next to the menu ("Plugins" -> "Debugger" -> "Toggle Breakpoint"), so when people get bored of three layer menu inception, they can learn to use the hot key. Though I am not sure how much it is gonna help