Better support for multiple keyboard layouts.

[tomboehmer]

Disclaimer: I am opening this pull request to get some early feedback on my changes. I am actually still on xbindkeys at the moment, so I have not even fully tested them (ctrl + u: echo hello word works, though). There are some todos I would like your input on.

How I want to handle multiple layouts

• specify a base layout up front (command line / maybe config file)
• resolve key codes with respect to this layout regardless of the one being active

How this is different from the status quo

• as I understand it, sxhkd currently locks in on the layout that is active at startup (or responds to mapping events)
• I never actually use the layout I want as my base (base: qwertz, active: dvorak,cyrillic,...)
• current mapping can be broken by restarting sxhkd while the wrong initial layout is active

Status

• Use XKB to select the keymap and handle keycode <-> keysym conversions
• Treat ctrl + U as ctrl + shift + u (what exactly is the rule here, is there a point to it anyway?)
• Figure out the point of the keysym -> keycodes -> keysyms conversion sequence (does this relate to the above point?)
• Add the command line flags (--rule,--model,--layout,--variant,--options)