Skins2: Add (and use) template parameter typedefs so that they might be used, say, as function return types. The next commit needs this.

Skins2: Factor out duplicate code (mod), and provide a const accessor to the keymap: std::map<>::operator[] will create empty nodes if they don't exist already.

Skins2: x11_display: Replace PUT_PIXEL and BLEND_PIXEL macros with templates. Could use more testing and possibly more refactoring.

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.