desktop-shell: Always activate the top-level surface

In case the device has only touchscreen input device and no keyboard or mouse, the top level surface is never activated. The behavior differs from a device which has a keyboard (or gpio-keys, or even uinput-emulated keyboard), where callchain activate()->weston_view_activate()->weston_seat_set_keyboard_focus()-> weston_keyboard_set_focus()->wl_signal_emit(&keyboard->focus_signal, keyboard)-> handle_keyboard_focus()->weston_desktop_surface_set_activated(..., true); sets the top level surface as activated. On device with touchscreen, the above is never called, hence the top level surface is never activated. Add explicit weston_desktop_surface_set_activated(shsurf->desktop_surface, true); into activate() to always active the top level surface.

This fixes at least two known issues on such devices:

• Wayland terminal cursor is an empty bar (full bar with keyboard present)
• Chromium dropdown menus are randomly placed (they are placed correctly when keyboard is present, because then chromium can find the activated top level surface)

Superseded by !570 (merged)