Bug 329449 – [GTK][WPE][WebDriver][BiDi]: support the Back and Forward mouse buttons

• Status: NEW
• Component: WebDriver

[Razvan Caliman]

WebDriver button 3 (Back) and button 4 (Forward) are supported only on Mac as of bug 326378. 

On other ports, the platform mouse-button mappings send them as a different button:
  - GTK: `mouseButtonToGdkButton` in `gtk/WebAutomationSessionGtk.cpp` sends them as the middle button.
  - WPE: `mouseButtonToWPEButton` in `libwpe/WebAutomationSessionLibWPE.cpp` and `mouseButtonToWPEPlatformButton` in `libwpe/WebAutomationSessionWPE.cpp` send them as the primary button.

Each of these functions has a "FIXME: Add support for the forward and backward mouse buttons."

Until this is fixed, BiDi `input.performActions` returns "unsupported operation" for these buttons on non-Mac ports.

Fix:

1. Map MouseButton::Back and MouseButton::Forward to the native buttons: GDK buttons 8 and 9, and the matching WPE/libwpe values.
2. Remove the guard in `mouseButtonForW3CButton` in `BidiInputAgent.cpp`
  
Tests: bidi/input/perform_actions/pointer_mouse.py and the classic perform_actions tests that use buttons 3 and 4.