Skins2: Use vlc KEY_MODIFIER_* constants for key modifier masks, simplifying the code. Revert from enum to static const int to not have to include vlc_keys.h through the header file.

Skins2: Factor out getModKey() and getKeyState() (both used twice), use var_SetVariant() instead of var_Set().

Skins2: Refactor string manipulation event handling a bit and replace some static const members with an enum.

Skins2: Use typedefs for internal template-derived types, and rectify an accidental skip while removing.

Add a GetColumnSize function in the SQL API.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>