lib: Timer: Add startMs function.
lib: KeyEvent: Correctly initialize m_key.
lib: Add initial mouse support.

Current implementation doesn't work on VT's. It seems like it can be only
done using libgpm. Ncxmms2 uses libtermkey for mouse support, so it can
regarded as libtermkey bug, and should be fixed there.
lib: LineEdit: Add mouse support.
lib: TextView: Add mouse support.
lib: ListView: Add mouse support.
Settings: Add mouse related settings.
MainWindow: Forward mouse events to StackedWindow or StatusArea only.
MedialibBrowser: Add mouse support.
PlaylistsBrowser: Add mouse support.
PlaybackProgressBar: Add mouse support.