Remove synclient backend
Remove plugin-loading code

I think that there won't be multiple backends in near future.
Also, it looks like KDE's plugin system is an overkill here.
Remove "test" method from TouchpadBackend
Xlib backend: try to report errors
touchpad.kcfg: "Fix" TapButton* parameters
Set SetUserTexts=true
Set labels for choices
Populate comboboxes with choices from KConfig