wpesrc: Add support for modifiers in key/touch/pointer events
Implement the new APIs from !2010 (merged) for wpesrc so that it can receive the modifiers.
This also fixes a bug where the wpe_input_event_keyboard was used uninitialized and it caused WPE to get random modifiers, and if you're unlucky, the key events appear broken.
Edited by Olivier Crête